Two workers at Japan's damaged Fukushima Daiichi nuclear power plant have been taken to hospital after being exposed to high levels of radiation.
The pair had been attempting to restore the cooling system in reactor 3, which was damaged by the quake on 11 March.
Several workers have now been hurt on the site, an indication of the scale of the task facing them.
Radiation levels in Tokyo's water supply have now fallen, but remain high in other areas of northern Japan.
The official death toll from the magnitude 9.0 quake and subsequent tsunami has now risen to 9,523. Another 16,094 people are listed as missing.
Japan's nuclear safety agency said three workers had been injured when their feet came into contact with radiation-contaminated water while laying cables in the turbine area of reactor 3.
They were exposed to radiation levels of 170-180 millisieverts, he said, which is lower than the maximum level permitted for workers on the site of 250 millisieverts. Two of the workers were taken to hospital.
"Although they wore protective clothing, the contaminated water seeped in and their legs were exposed to radiation," said a spokesman.
"Direct exposure to radiation usually leads to inflammation and so that's why they were sent to the hospital to be treated."
Most people are exposed to 2 millisieverts over the average year, while 100 millisieverts is considered the lowest level at which any increase in cancer is clearly evident.
The condition of the injured workers was not immediately known.
Japan's chief cabinet secretary Yukio Edano said the situation was "very regrettable".
The power plant's cooling systems failed after the quake and tsunami, leading to the reactors overheating.
Power has now been restored to the site, but work to restart the coolers in reactor 3 was briefly suspended on Wednesday after a plume of black smoke was seen coming from it.
Tokyo Electric Power Co, which operates the plant, later allowed workers to re-enter after establishing there was no fire and that radiation level in the area had not risen.
The International Atomic Energy Agency (IAEA) said there had been some "positive developments" at the site but that the situation was still "of serious concern".
The plant is 250km (155 miles) north-east of the capital, Tokyo. The government has declared a 20km exclusion zone and evacuated tens of thousands of people. Those living up to 30km away have been told to stay indoors to minimise exposure.
People in Fukushima prefecture have been told not to eat 11 types of green leafy vegetables grown locally because of contamination worries. Local producers have been ordered not to send the goods to market.
Tokyo residents were warned on Wednesday not to give tap water to babies less than a year old because levels of radioactive iodine - which can cause thyroid cancer - are twice the recommended safe level in some areas of the city.
Officials stressed that children would have to drink a lot of it before it harmed them and urged people not to panic-buy. But supermarket shelves were reported to have been cleared of bottled water by Thursday morning.
"Customers ask us for water. But there's nothing we can do," Masayoshi Kasahara, a supermarket worker in Tokyo told Reuters.
"We are asking for more deliveries but we don't know when the next shipment will come."
Emergency shelters
Radiation readings on Thursday showed levels in water in Tokyo had fallen back below the danger level, but the municipal authorities are distributing thousands of bottles of water to households with infants.
The authorities in the nearby city of Kawaguchi, Saitama prefecture, also reported radiation levels above safety norms in its water supply on Thursday.
Concern is also growing among Japan's neighbours. Australia has become the latest country to ban food imports from the affected region.
Police believe the final death toll from Japan's twin disaster may be more than 18,000.
Most of the deaths - 5,700 - have been reported in the prefecture of Miyagi. Three thousand bodies have been found in Iwate prefecture, and 776 in Fukushima.
At least 18,000 houses were destroyed and 130,000 damaged, and more than 200,000 people are living in emergency shelters.
The Japanese government has said it will cost as much as 25 trillion yen ($309bn; £189bn) to rebuild the country after the disaster.
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Fighting between rebels and forces loyal to Libyan leader Col Muammar Gaddafi has been continuing in key cities after a fifth consecutive night of air strikes by international forces.
Overnight, several loud explosions were heard in the capital, Tripoli.
In the rebel-held city of Misrata, east of Tripoli, government tanks have been shelling the area near the hospital.
A French fighter has shot down a Libyan air force jet which was violating the no-fly zone, ABC News has reported.
There have also been reports of fierce fighting between rebels and pro-Gaddafi forces in strategic Ajdabiya.
Residents fleeing the town described shelling, gunfire and houses on fire.
Air strikes
There are reports of Western air strikes hitting the Tajura district of the capital, Libyan military sources and media have said.
And the French military has said French air strikes hit a Libyan air base in the interior of the country overnight.
The strikes hit a base about 250km (155 miles) south of the Libyan coastline, French military spokesman Col Thierry Burkhard told reporters. He did not give any further information on the location of the target or the damage.
Later on Thursday, Western military planes were reported to have hit the town of Sebha in southern Libya, according to residents and media reports.
Sebha, about 750km (480 miles) south of Tripoli, is a Gaddafi stronghold and home to an important military base.
An explosion was also reported overnight at a military base in the Tajura region east of Tripoli. Residents in the capital earlier said plumes of black smoke could be seen coming from an area near a military base, although this has not been independently confirmed.
Tanks return
In Misrata, Libya's third-largest city, witnesses had said on Wednesday that tanks had pulled back from their positions under air assault from international forces.
But later, residents said the tanks had rolled back into the city and resumed shelling.
Misrata resident Muhammad told the BBC many large explosions were heard overnight in the city: "Even now we continue to hear the aeroplanes circling the air above Misrata right now...
"Our major problem in Misrata is with the snipers. Gaddafi's forces have occupied the main street... which stretches from the town centre all the way to the highway and beyond. There are snipers all along the rooftops of that street. They are firing indiscriminately into the main street and the back streets. But the heavy artillery and shelling has stopped since yesterday [Wednesday]. In that sense, we are in a much better position."
Another resident has said pro-government forces have seized control of the city's port, where there are thousands of stranded foreign workers seeking to leave, Reuters news agency reported on Thursday.
The same resident has said Western air strikes hit government tanks on the outskirts of the city late on Wednesday, but tanks inside the city remain there and have not been hit.
Misrata has been besieged for several weeks, with reports that more than a dozen people were killed in the latest clashes.
John SimpsonWorld Affairs Editor, BBC News, Tripoli
After one particularly loud explosion during the night, a column of smoke was seen rising from the direction of a military base in the city. There have been various bursts of prolonged firing from the ground, completely useless in terms of hitting the planes or missiles involved, but often an indication that the defenders think they are coming under attack.
There are contradictory accounts about the effectiveness of coalition attacks on Col Gaddafi's forces outside the rebel-held town of Misrata. On Wednesday, pro-Gaddafi tanks seemed close to taking the town, but after a number of coalition air strikes, they appear to have been silenced. Still, there are suggestions that the tanks and artillery may have opened up again under cover of darkness.
Misrata is the last major town in western Libya that's been holding out against the pro-Gaddafi counter attack, so it's important from the coalition's point of view to protect it from falling.
It may take the coalition of international powers days or weeks to destroy Col Gaddafi's military, but it will not require months, French Foreign Minister Alain Juppe has told reporters.
Earlier, the US chief of staff for the mission in Libya insisted there had been no reports of civilian casualties caused by allied action.
Rear Admiral Gerard Hueber's comments came despite claims to the contrary by Col Gaddafi's government.
Operational control
Earlier, British Air Vice Marshal Greg Bagwell said Col Gaddafi's air force no longer existed as a fighting force.
AVM Bagwell said the allies could now operate "with near impunity" over the skies of Libya and were now applying unrelenting pressure on the Libyan armed forces.
"We are watching over the innocent people of Libya and ensuring that we protect them from attack," he said. "We have the Libyan ground forces under constant observation and we attack them whenever they threaten civilians or attack population centres."
His comments came as Nato members debated who should lead the intervention, with the US keen to hand over operational control to Nato.
Nato members have been holding talks about assuming responsibility for the no-fly zone over Libya, so far without agreement.
Turkey is an integral part of the naval blockade, but has expressed concern about the alliance taking over command of the no-fly zone from the US.
US Secretary of State Hillary Clinton has again urged Col Gaddafi to step down and leave Libya.
UN Secretary General Ban Ki-moon has urged all sides in Libya to cease hostilities. "All those who violate international humanitarian and human rights law will be held fully accountable," his spokesman Martin Nesirky said.
US Defence Secretary Robert Gates is in the Egyptian capital, Cairo, for talks on both Libya and Egypt's hoped-for transition to democracy following the fall of Hosni Mubarak.
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Fighting between rebels and forces loyal to Libyan leader Col Muammar Gaddafi has been continuing in key cities after a fifth consecutive night of air strikes by international forces.
Overnight, several loud explosions were heard in the capital, Tripoli.
In the rebel-held city of Misrata, east of Tripoli, government tanks have been shelling the area near the hospital.
A French fighter has shot down a Libyan air force jet which was violating the no-fly zone, ABC News has reported.
There have also been reports of fierce fighting between rebels and pro-Gaddafi forces in strategic Ajdabiya.
Residents fleeing the town described shelling, gunfire and houses on fire.
Air strikes
There are reports of Western air strikes hitting the Tajura district of the capital, Libyan military sources and media have said.
And the French military has said French air strikes hit a Libyan air base in the interior of the country overnight.
The strikes hit a base about 250km (155 miles) south of the Libyan coastline, French military spokesman Col Thierry Burkhard told reporters. He did not give any further information on the location of the target or the damage.
Later on Thursday, Western military planes were reported to have hit the town of Sebha in southern Libya, according to residents and media reports.
Sebha, about 750km (480 miles) south of Tripoli, is a Gaddafi stronghold and home to an important military base.
An explosion was also reported overnight at a military base in the Tajura region east of Tripoli. Residents in the capital earlier said plumes of black smoke could be seen coming from an area near a military base, although this has not been independently confirmed.
Tanks return
In Misrata, Libya's third-largest city, witnesses had said on Wednesday that tanks had pulled back from their positions under air assault from international forces.
But later, residents said the tanks had rolled back into the city and resumed shelling.
Misrata resident Muhammad told the BBC many large explosions were heard overnight in the city: "Even now we continue to hear the aeroplanes circling the air above Misrata right now...
"Our major problem in Misrata is with the snipers. Gaddafi's forces have occupied the main street... which stretches from the town centre all the way to the highway and beyond. There are snipers all along the rooftops of that street. They are firing indiscriminately into the main street and the back streets. But the heavy artillery and shelling has stopped since yesterday [Wednesday]. In that sense, we are in a much better position."
Another resident has said pro-government forces have seized control of the city's port, where there are thousands of stranded foreign workers seeking to leave, Reuters news agency reported on Thursday.
The same resident has said Western air strikes hit government tanks on the outskirts of the city late on Wednesday, but tanks inside the city remain there and have not been hit.
Misrata has been besieged for several weeks, with reports that more than a dozen people were killed in the latest clashes.
John SimpsonWorld Affairs Editor, BBC News, Tripoli
After one particularly loud explosion during the night, a column of smoke was seen rising from the direction of a military base in the city. There have been various bursts of prolonged firing from the ground, completely useless in terms of hitting the planes or missiles involved, but often an indication that the defenders think they are coming under attack.
There are contradictory accounts about the effectiveness of coalition attacks on Col Gaddafi's forces outside the rebel-held town of Misrata. On Wednesday, pro-Gaddafi tanks seemed close to taking the town, but after a number of coalition air strikes, they appear to have been silenced. Still, there are suggestions that the tanks and artillery may have opened up again under cover of darkness.
Misrata is the last major town in western Libya that's been holding out against the pro-Gaddafi counter attack, so it's important from the coalition's point of view to protect it from falling.
It may take the coalition of international powers days or weeks to destroy Col Gaddafi's military, but it will not require months, French Foreign Minister Alain Juppe has told reporters.
Earlier, the US chief of staff for the mission in Libya insisted there had been no reports of civilian casualties caused by allied action.
Rear Admiral Gerard Hueber's comments came despite claims to the contrary by Col Gaddafi's government.
Operational control
Earlier, British Air Vice Marshal Greg Bagwell said Col Gaddafi's air force no longer existed as a fighting force.
AVM Bagwell said the allies could now operate "with near impunity" over the skies of Libya and were now applying unrelenting pressure on the Libyan armed forces.
"We are watching over the innocent people of Libya and ensuring that we protect them from attack," he said. "We have the Libyan ground forces under constant observation and we attack them whenever they threaten civilians or attack population centres."
His comments came as Nato members debated who should lead the intervention, with the US keen to hand over operational control to Nato.
Nato members have been holding talks about assuming responsibility for the no-fly zone over Libya, so far without agreement.
Turkey is an integral part of the naval blockade, but has expressed concern about the alliance taking over command of the no-fly zone from the US.
US Secretary of State Hillary Clinton has again urged Col Gaddafi to step down and leave Libya.
UN Secretary General Ban Ki-moon has urged all sides in Libya to cease hostilities. "All those who violate international humanitarian and human rights law will be held fully accountable," his spokesman Martin Nesirky said.
US Defence Secretary Robert Gates is in the Egyptian capital, Cairo, for talks on both Libya and Egypt's hoped-for transition to democracy following the fall of Hosni Mubarak.
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A coalition of Western Allies has launched a series of airstrikes against military targets in Libya. US and British warships and submarines in the Mediterranean, as well as British, US and French fighter jets, took part in the raids.
More fighters are now being moved to forward positions, closer to Libya. British jets are flying down to southern Italy. Danish and Canadian jets are on standby in Corsica.
Pro-Gaddafi forces have attacked the Libyan rebel stronghold of Benghazi and their tanks have entered the city, a BBC journalist witnessed.
A jet also appears to have been shot down over the city despite a declared ceasefire and a UN no-fly resolution.
World leaders are meeting in Paris to discuss military action.
The rebel's leader has appealed to the international community to stop the pro-Gaddafi bombardment, but the government denies it is attacking.
Reports suggest hundreds of cars packed with people were fleeing the city eastwards as fighting spread.
"Now there is a bombardment by artillery and rockets on all districts of Benghazi," rebel leader Mustafa Abdul Jalil told Al Jazeera television. "There will be a catastrophe if the international community does not implement the resolutions of the UN Security Council.
UN Secretary General Ban Ki-moon says the world must "speak with one voice" on Libya.
The new UN resolution authorised "all necessary measures" to protect Libyan civilians.
Getting ready
The BBC's Ian Pannell saw the government tanks on a bridge inside Benghazi at around 1030 (0830 GMT).
Earlier, US President Barack Obama said forces loyal to Col Muammar Ghaddafi must stop attacking rebel areas or face military action.
"Gaddafi must stop his troops from advancing on Benghazi, pull them back from Ajdabiya, Misrata and Zawiya and establish water, electricity and gas supplies to all areas," he said on Friday.
Benghazi residents were woken up at 0530 by sounds of what rebels say was a bombardment of city.
Shortly after 0900 a fighter jet was hit - it is unclear by what, and we don't know who it belongs to, but it caught fire and plummeted to earth, where it exploded.
The plane hit the ground in the western side of Benghazi, bursting into flames. A huge pall of smoke is currently rising over the city.
It would seem that Col Gaddafi's announcement of a ceasefire has been completely ignored by his troops.
Diplomatic sources suggest world powers could start launching air-strikes on Libya within hours.
On Friday, Col Gaddafi's government declared a unilateral truce but there were reports that government offensives in rebel-held towns were continuing, and our correspondent says gunfire was also heard coming from the sea.
A rebel spokesman was quoted as saying the downed jet was a rebel plane which had been shot down by pro-government forces.
The United Nations refugee agency says it is preparing to receive 200,000 people fleeing the fighting, amid reports of hundreds of cars full of people heading for the Egyptian border, while others are attempting to flee on foot.
The first families had arrived at the Egyptian border, extremely frightened and traumatised, saying some of their homes have been completely flattened said UNHCR spokeswoman Elizabeth Tan.
While officially, Libya's government claims it is respecting its ceasefire, on the ground a darker reality is emerging, says the BBC's Kevin Connolly in the eastern city of Tobruk:
Col Gaddafi may be using the interlude being the UN resolution being passed and its being implemented to punish rebels for their uprising before the West can intervene
The Libyan leader may also be seeking to conceal his armoured columns in the streets of Benghazi, making it infinitely harder for allied forces to attack them
Second largest city in Libya with population of 670,000
Industrial centre important to Libya's economy due to imports of foodstuffs and manufactured products
Heavily bombed in World War II; later rebuilt with country's newly found oil wealth as a showpiece for modern Libya
Scene of violent anti-government protests last month; announced as HQ of rebel transitional council on 26 February
But a Libyan government spokesman said there had been "no attacks whatsoever on Benghazi".
"As we said, we are observing the ceasefire and we want international observers to come," said the spokesman, Mussa Ibrahim.
Mr Ibrahim quoted a letter from Col Gaddafi to President Sarkozy of France, UK Prime Minister David Cameron and Mr Ban, saying the UN resolution had no authority because it was interfering in Libya's internal affairs.
The Security Council vote called for an immediate ceasefire, and endorsed using force and imposing a no-fly zone to protect civilians.
The British and French, along with some Arab allies, are expected to play a leading role in the enforcement of the resolution.
The French ambassador to the UN, Gerard Araud, told the BBC that he expected military intervention within hours of Saturday's summit.
Mr Cameron - who is scheduled to meet Mr Sarkozy and US Secretary of State Hillary Clinton ahead of the summit - has said British war planes are being moved to bases in the region.
Strength of outrage
Saturday's summit will be hosted by Mr Sarkozy. Mrs Clinton will represent the Obama administration while Arab nations expected to be represented include Jordan, Morocco, Qatar and the United Arab Emirates.
BBC diplomatic correspondent Bridget Kendall says it is a chance to demonstrate to the Gaddafi government, and to the rest of the world, the strength of outrage at his actions and the determination to stop him.
The US Navy is deploying additional warships to the Mediterranean to support possible military action.
Click to play
President Obama: Colonel Gaddafi has received "ample warning"
The commander in charge of planning for the no-fly zone will be US Adm Samuel Locklear, the BBC's Mark Urban has learnt.
Denmark and Canada have said they will supply fighter jets, with Italy, Spain and France making air bases available.
Nato is already providing 24-hour surveillance over Libya with its Awacs planes.
Secretary-General Anders Fogh Rasmussen said Nato was "completing its planning to be ready to take appropriate action in support of the UN resolution as part of the broad international effort."
The US ambassador to the UN, Susan Rice, told CNN that Col Gaddafi was already in violation of the UN Security Council resolution.
Col Gaddafi has ruled Libya for more than 40 years.
An uprising against him began last month after long-time leaders of neighbouring Tunisia and Egypt were toppled.
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THERE are many similarities between Libya and Iraq. The landscape is crusty and pancake flat. The housing is similarly spartan, and relentlessly dun. Because of a history of sanctions and the caprice of their leadership, Libyans and Iraqis alike have benefitted little from their country’s oil wealth. Both suffer from neglect.Last night's UN Security Resolution 1973 adds another parallel: the imposition of a no-fly zone and authorisation of "all necessary means" to protect civilians. In Libya, that means strikes on Colonel Muammar Qaddafi's ground forces if they start to shell towns. It also sanctions ground attacks and strikes on Libyan naval vessels if they too are used for bombardment. France has suggested that airstrikes against the Libyan leader's forces could start within hours. David Cameron, the British prime minister, announced on Friday that British Tornado and Typhoon jets are being moved into place in readiness for any action in Libya.
In response to the resolution, the Libyan government has said it is declaring a ceasefire and calling a halt to all military operations, in accordance with the resolution's dictates about the protection of civilians. How much that really means remains to be seen. Both Britain and France have said that they will judge Colonel Qaddafi on his actions, not on his words. Nonetheless, a ceasefire may make it harder to justify bombing him.
In April 1991, the UN Security Council passed a resolution calling on Iraq to end the repression of its population, and America, Britain and France imposed a no-fly zone. It was already too late though for Iraq's southern Shia, who despite western support for their uprising, were left to the ravages of the Iraqi Republican Guard, and killed in their thousands. But the Kurds in the north gained their own autonomous safe haven.
As in Saddam Hussein’s Iraq, the UN decision comes too late for many Libyans. Libya's western towns have been muzzled by the colonel’s counter-attack. It will, however, bring relief to the east. Real or imagined, the fear of a chemical weapons drop that made easterners tremble has receded. Libyan families in the east who feared another Halabja, the town Saddam Hussein's MiG aircraft gassed in March 1988, can breathe more easily.
A no-fly zone also significantly redresses the balance in the conflict between the regime and the rebels. In a vast country four times the size of Iraq, air capability is critical to Tripoli's projection of central power. Even while bunkered in his Bab al-Aziziya military base, the colonel maintained his strategic depth. It enabled him to move troops and maintain supply lines across the oil-rich Sahara separating east from west. Without his aerial reach, his forces are as vulnerable to overstretch as the rebels.
On the battlefield, too, air power gave the colonel the tactical edge. Each time rebel and regime infantries clashed, air power was decisive. In the flat barren terrain, it enabled the colonel to observe and then bomb exposed rebel lines, sending his ragtag opposition positioned on main roads into retreat. Aerial bombardment, too, prevented ill-trained irregulars from regrouping each time they tried to mount an effective counter-attack.
Psychologically, the UN's endorsement of a no-fly zone (something the Kurds never had) is a major boon for the rebels. As the colonel's forces advanced and the international community appeared to dither, many Libyans who had thrown in their lot with the rebels began to waver. In Benghazi, the main rebel town, businessmen who had strung rebel flags from their balconies or stuck them on their cars quietly took them down. The colonel's revolutionary-committee loyalists resurfaced in rebel-held areas, demanding pledges of allegiance from drivers trapped in traffic. Powerful tribal leaders in central Libya whose support could prove pivotal sat on the fence. Just as the colonel's curtain of fear had begun to descend once again, the promise of international protection will hoist it back up.
The danger remains of western military mission creep. Colonel Qaddafi might yet respond by lashing out wildly. In a television interview, he threatened to act as crazily as the external military forces stacked against him; and his defence-ministry spokesman suggested Tripoli might respond by attacking air or maritime traffic in the Mediterranean. But Iraqi precedent suggests otherwise. Saddam Hussein abided by the terms of a no-fly zone and grounded his planes, rather than risk drawing the west into a deeper conflict that might unseat him.
That said, a no-fly zone is by itself unlikely to deliver a rapid denouement, or achieve a cessation of violence. On the ground and at sea—thanks to Italy’s provision of speedboats—the colonel’s better trained, paid and armed troops still have the edge. They retain control of the west and much of the centre of the country, and from their frontline at Ajdabiya, the gateway to Benghazi, continue to cast a shadow over the rebel’s eastern holdings and the southern huge oil-fields. For their part, the rebels, hopeful of a more even fight, might use the reduced threat of aerial bombardment to redouble their efforts to march on Tripoli. Far from ending the conflict, the no-fly zone might extend the ground war amongst the oil installations and along well-trodden desert lines. Without or without mediation, the de facto division of Libya into an autonomous eastern safe haven and embittered West could be acquiring a more concrete form.
Libya's government is declaring an immediate ceasefire, hours after a UN Security Council resolution backed a no-fly zone over the country.
Libyan Foreign Minister Mussa Kussa said the ceasefire was intended "to protect civilians".
The UN resolution supported "all necessary measures" to protect civilians, short of an occupation.
Western powers had been discussing how to enforce the no-fly zone.
Before the announcement of the ceasefire, fighting between troops loyal to Libyan leader Col Muammar Gaddafi and rebel forces was reported to be continuing.
Japan's upgrading of the Fukushima incident from severity four to five stems from concerns about the reactors in buildings 1, 2 and 3, rather than the cooling ponds storing spent fuel.
Level five is defined as an "accident with wider consequences". This was the level given to the 1957 reactor fire at Windscale in the UK and the partial meltdown of the Three Mile Island plant in the US in 1979.
Both met the level five definition of "limited release" of radioactive materials to the wider environment.
Windscale is believed to have caused about 200 cases of cancer, whereas reports into the Three Mile Island incident suggest there were no health impacts outside the site.
French and US officials had previously said the Fukushima situation was more serious than Japanese evaluations suggested.
Higher radiation levels than normal have been recorded in a few places 30km from the site, but in Tokyo, they were reported to be normal.
Despite official assurances that the radiation risk is virtually nil outside the 20-km (12-mile) exclusion zone around the plant, unease has spread overseas.
Britain and the US are among countries which have organised aircraft to evacuate from Japan those of their citizens who are concerned.
Store shelves in parts of the US have been stripped of iodine pills, which can protect against radiation, while Asian airports have been scanning passengers from Japan for possible contamination.
Shoppers in China have been panic-buying salt in the mistaken belief that it can guard against radiation exposure.
Meanwhile, Prime Minister Naoto Kan told a national television address: "We will rebuild Japan from scratch. We must all share this resolve."
He said the natural disaster and nuclear crisis were a "great test for the Japanese people", but exhorted them all to persevere.
The head of the International Atomic Energy Agency, Yukiya Amano, arrived earlier in Tokyo and warned the Fukushima crisis was a "race against the clock".
"This is not something that just Japan should deal with, and people of the entire world should co-operate with Japan and the people in the disaster areas," said Mr Amano, a Japanese citizen.
He said he would not visit the Fukushima Daiichi site, which has been rocked by a series of explosions, on his current trip to the country.
His four-member team of nuclear experts would start by monitoring radiation in the capital, he said, before moving to the vicinity of the quake-hit facility.
Military fire trucks have been spraying the plant's overheating reactor units for a second day.
Water in at least one fuel pool - reactor 3 - is believed to be dangerously low, exposing the stored fuel rods.
If the ponds run dry, a nuclear chain reaction could release more radiation into the atmosphere.
An electricity line has been bulldozed through to the site and engineers are racing to connect it, but they are being hampered by radiation.
The plant's operators need the power cable to restart water pumps that pour cold water on the reactor units.
Military helicopters which dropped water from above on Thursday have been kept on standby.
Televised footage of the airdrops had shown much of the water blowing away in the wind.
The first US government-chartered evacuation flight has taken off from quake-hit Japan for Taiwan, amid rising fears over the country's stricken Fukushima nuclear power plant.
The plane carried 100 Americans, mostly relatives of government workers.
France and China are also to evacuate thousands, while the UK and Australia have advised citizens to leave Tokyo.
President Barack Obama said harmful radiation was not expected to reach the US or its Pacific territories.
"We have seen an earthquake and tsunami render an unimaginable toll of death and destruction on one of our closest friends and allies in the world," Mr Obama said on Thursday at the White House.
"And we've seen this powerful natural disaster cause even more catastrophe through its impact on nuclear reactors that bring peaceful energy to the people of Japan."
'Exhaustive review' pledge
Mr Obama also advised Americans not to take precautionary measures against radiation other than to stay informed; runs on potassium iodide pills, seen as a defence against radiation poisoning, have been reported in a number of countries.
US - providing flights for people who wish to leave, advising 50-mile exclusion zone around Fukushima
France - urging people to leave northern Japan and Tokyo, sending government planes to fly French out
UK - advising nationals to leave north-east and Tokyo, chartering flights out
China - bringing thousands to Tokyo for evacuation
Australia - people with non-essential roles to leave Japan
And Mr Obama pledged an "exhaustive review" of the US nuclear power infrastructure.
"Nuclear energy is an important part of our own energy future," he said.
Mr Obama has assured Japan of America's ongoing support, and the US has sent food aid and military personnel to back Japanese efforts, and is sending nuclear and biological hazard specialists.
Meanwhile, Australian Foreign Minister Kevin Rudd advised Australians to leave Japan, China has moved thousands of people to Tokyo to prepare for their evacuation, and France has assigned two government planes to evacuate its nationals.
Most other countries have also advised their nationals to leave the north-eastern region of Japan or to leave the country altogether.
UK Foreign Office said that any Britons living in or north of Tokyo should "consider leaving", but officials have stressed there is no immediate risk to health.
US Under Secretary of State Patrick Kennedy said another US evacuation flight was scheduled to leave Japan on Friday, AFP news agency reported.
Customs screen
Thousands of evacuees have crowded Narita airport in hopes of leaving Japan
US customs agents will screen passengers and cargo from Japan for "even a blip of radiation", Homeland Security Secretary Janet Napolitano said on Thursday.
She called the screening an "exercise of caution".
The US state department warned Americans to delay travel to Japan, and saying unpredictable weather may spread radiation in the areas north of Tokyo.
It has offered voluntary evacuations to about 600 family members of state department personnel in Tokyo, Nagoya and Yokohama.
The US military has 55,000 troops in the Japan region, who have about 43,000 dependants. Thousands of other government and private sector employees also reside there.
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The governor of the region at the centre of Japan's nuclear crisis has criticised official handling of the evacuation of the area around the stricken Fukushima Daiichi power plant.
Fukushima prefecture governor Yuhei Sato said: "Anxiety and anger felt by people have reached boiling point."
Engineers are racing to avert a nuclear catastrophe at Fukushima Daiichi, badly damaged by Friday's quake and tsunami.
The government has declared a 20km (12-mile) evacuation zone around it.
Another 140,000 people living between 20-30km of the facility were told on Tuesday not to leave their homes, while the US embassy has advised American citizens living within 80km of the plant to evacuate or seek shelter.
Mr Sato said centres already housing people who had been moved from their homes near the plant did not have enough hot meals and basic necessities such as fuel and medical supplies. "We're lacking everything," he said.
Japanese media have became more critical of Prime Minister Naoto Kan's handling of the disaster, and have accused both the government and plant operator Tokyo Electric Power Co of failing to provide enough information on the incident.
Over the days of the Fukushima crisis, attention has switched from reactor building 1 to 3, to 2, back to 3 - and now, to 4.
Here, it is not the actual reactor that is causing concern. Instead, it is a pool storing fuel rods that had been taken out of the reactor when it was shut down for maintenance before the earthquake struck.
There have been reports that water levels were low; and now the US Nuclear Regulatory Commission (NRC), which has a team of experts advising in Japan, says the pool is completely dry.
This means the fuel rods are exposed to the air. Without water, they will get much hotter, allowing radioactive material to escape - and the NRC says radiation levels are probably extremely high, creating a danger to workers at the plant.
The company operating the plant has even warned of 're-criticality' - that a nuclear chain reaction could start among fuel rods in the now dry pool.
That would not cause a nuclear explosion but it would increase the release of radioactive substances.
Thousands of people were killed in the 9.0-magnitude quake and tsunami. In a rare public appearance, Japan's Emperor Akihito has said he is "deeply worried" about the crisis his country is facing.
The atomic crisis has been caused by the tsunami wrecking back-up diesel generators which kept the nuclear fuel cool at the Fukushima Daiichi plant, 220km from Tokyo.
Workers have been dousing the reactors with seawater in a frantic effort to stabilise their temperatures, since the first in a series of explosions rocked the plant on Saturday.
Helicopters deployed to dump water on the facility on Wednesday were pulled out amid concerns over radiation levels in the air above the site. Reports suggest another plan is now under consideration to use water cannon.
Earlier, the plant's operators evacuated its skeleton crew of 50 workers for about an hour as ground-level radiation spiked.
And yet another fire broke out in a reactor, while steam billowed from another one.
The power facility has sent low levels of radiation wafting into Tokyo, spreading alarm in the city and internationally.
The head of the United Nations nuclear watchdog, Yukiya Amano, said developments at the plant were "very serious", as he prepared to head to the country to assess the situation.
In other developments:
After losing $620bn (£385bn) in the first two days of this week, Japan's stock market rebounded to finish Wednesday up by 5.7%
Britain advised its nationals currently in Tokyo and to the north of the capital to consider leaving the area
France urged its nationals in Tokyo to leave the country or move south; two Air France planes were sent to begin evacuation
Australia advised its citizens to consider leaving Tokyo and the most damaged prefectures
Turkey warned against travel to Japan
'Unprecedented'
Emperor Akihito went on live TV on Wednesday to make his first public comments on the disaster, and urged an all-out rescue effort.
Watch: Japan's Emperor Akihito addresses the Japanese people
TV stations interrupted programming to show the emperor describing the crisis facing the nation as "unprecedented in scale".
The 77-year-old - deeply respected by many Japanese - said: "I hope from the bottom of my heart that the people will, hand in hand, treat each other with compassion and overcome these difficult times."
Japan's titular head of state - who acceded to the throne in 1989 after the death of his father Hirohito - said he prayed that every victim would be saved.
He spoke as snow blanketed swathes of the disaster zone, where many survivors have little food, water or heat.
About 450,000 people have been staying in temporary shelters, many sleeping on the floor of school gymnasiums.
More than 4,300 people are listed as dead but it is feared the total death toll from the catastrophe, which pulverised the country's north-east coast, will rise substantially.
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