by Natalie de Vallieres and Nasra Ismail, BBC News, for the BBC's Biodun Iginla 

ISTANBUL – The collapse of another attempt at international outreach to Iran on Saturday has left world powers with few options except to wait — and hope that the bite of sanctions will persuade Tehran to reconsider its refusal to stop activities that could be harnessed to make nuclear weapons.

But their patience could be tested. While the U.S. and others say that Iran already is suffering from the wide range of financial and trade sanctions, travel bans and other penalties imposed by the U.N., the U.S., the EU and others, the Islamic Republic shows no sign of bending.

Uranium enrichment lies at the heart of the dispute.

Low-enriched uranium — at around 3.5 percent — can be used to fuel a reactor to generate electricity, which Iran says is the intention of its program. But if uranium is further enriched to around 90 percent purity, it can be used to develop a nuclear warhead.

Iran came to the Istanbul talks with six world powers Friday declaring it would not even consider freezing uranium enrichment — and left the negotiations Saturday repeating the same mantra. Throughout two days of hectic meetings, it stubbornly pushed demands it must have known were unacceptable to the six — a lifting of sanctions and acceptance of its enrichment program before any further discussion of its nuclear activities.

"Both these preconditions are not the way to proceed," EU foreign policy chief Catherine Ashton declared — and added no new talks were planned.

Publicly, the U.S. and others nations concerned that Iran could turn its enrichment program toward making fissile warhead material say that troubles with enrichment have slowed that activity and left more time to persuade Iran to heed international concerns than thought just a year ago.

Israeli officials now talk of a three-year window — until 2014 — before Iran can make a bomb. That compares with projections of 2011 just three years ago.

U.S. Secretary of State Hillary Rodham Clinton told NBC's "Today" show earlier this week that the new Israeli estimates are "very significant." The delay, she said, "gives us more of a breathing space to try to work to prevent them from obtaining a nuclear weapon."

Two outside forces could account for any Iranian problems in enriching uranium — the increasing weight of U.N. and other sanctions meant to choke off raw materials needed to make and maintain the program, and the apparent havoc caused by the mysterious Stuxnet computer malware, which experts think was created by Israel or the U.S.

"Sanctions have had an impact. There are signs that Iran's nuclear program has slowed, so I think there is time and space for diplomacy," a senior U.S. administration official said after the Istanbul talks collapsed. He asked for anonymity in exchange for discussing the delicate issue.

While there is no talk for now of U.N Security Council new sanctions past a fourth set in June that target Iran's Revolutionary Guard, ballistic missiles and nuclear-related investments, there have been significant Western efforts to enforce present penalties.

Senior U.S. officials have been touring China, Japan, South Korea and the pro-western Arab nations to demand compliance with the U.N sanctions and the European Union, Canada, Australia and others have followed Washington's example in imposing their own restrictions on trade, financial transactions and other relations with Iran.

A diplomat from a delegations that met with Iran in Istanbul told the AP Saturday that EU outsiders Switzerland and Norway would formally adopt the EU sanctions next week. He asked for anonymity because he was not authorized to discuss the matter.

On the cyber front, Iran has acknowledged that Stuxnet hit "a limited number of centrifuges," saying its scientists discovered and neutralized the malware before it could cause any serious damage. The computer worm is assumed to have caused disruption of enrichment in November that temporarily crippled thousands of centrifuges at Natanz and experts say it could flare up again to do further damage.

Still there is no reason for complacency.

In a study shared this week with The Associated Press, the Federation of American Scientists notes impressive improvements in the performance of the Iranian machines that enrich uranium. The centrifuges are still underperforming but FAS says Iran last year appears to have increased their efficiency by 60 percent, giving it the technical capacity to produce enough material for a simple nuclear warhead in five months.

Few people think that Tehran is likely to provoke the world — and increase the likelihood of U.S. or Israeli attack — by kicking out International Atomic Energy Agency inspectors and re-calibrating its centrifuges to make such a device.

Manufacturing nuclear warhead material is only one step in making a weapon, and Olli Heinonen, who retired late last year as the IAEA deputy director general in charge of the agency's Iran file, calls the likelihood of such a "breakout scenario" a "suicidal mission" because of the likelihood of overwhelming international retribution.

Still, Iran seems determined to let nothing stop it from expanding enrichment, even starting a smaller program that is churning out material that can be turned into weapons-grade uranium much more quickly than its large stockpile of low-enriched uranium.

Neither "resolutions, sanctions, threats, computer virus nor even a military attack will stop uranium enrichment in Iran," Ali Asghar Soltanieh, Iran's envoy to the International Atomic Energy Agency, declared as the talks began.

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Continue reading the main storyThe scientists were targeted in separate attacks on their way to work

by Nasra Ismail, BBC News Middle East Desk, for the BBC's Biodun Iginla

An Iranian nuclear scientist has been killed and another wounded in two separate but similar attacks in the capital, Tehran.

The scientists were targeted by men on motorbikes who attached bombs to the windows of their cars as they drove to work, officials said.

The scientist killed has been named as Majid Shahriari.

President Mahmoud Ahmadinejad has accused "Western governments" and Israel of being behind the killing.

Another scientist was killed in a bomb blast at the beginning of the year.

Dr Shahriari was a member of the nuclear engineering department of Shahid Beheshti University in Tehran. His wife is said to have been injured in the attack.

The nuclear scientist injured in the second attack was named as Fereydoon Abbasi. His wife was also wounded.

According to the conservative news website Mashregh News, Dr Abbasi is "one of the few specialists who can separate isotopes" - a process that is crucial in the manufacture of uranium fuel for nuclear power stations and is also required for the creation of uranium-based nuclear weapons.

Dr Abbasi has also been a member of the Revolutionary Guards since the 1979 revolution, the website said.

At a news conference, President Ahmadinejad accused Western powers and Israel of being behind the murder.

"One can undoubtedly see the hands of Israel and Western governments in the assassination which unfortunately took place," he said, without specifying which Western governments.

He said the assassination would not stop Iran from pursuing its nuclear programmes.

Earlier, state television reported a similar claim by Iran's Interior Minister, Mostafa Mohammad Najjar, who accused US and Israeli intelligence services of killing the scientist.

"Mossad and the CIA are the enemies of Iranians and always seek to hurt this nation. They particularly want to stop our scientific progress," he said.

The head of Iran's Atomic Energy Organisation, Ali Akbar Salehi, who went to visit the surviving scientist in hospital, said he had a message for the country's enemies: "Do not play with fire".

The Iranian scientist killed in January this year, Masoud Ali Mohammadi, was said to be a nuclear scientist assassinated by counter-revolutionaries, Zionists and agents of the "global arrogance", Iranian media said at the time.

But scientists in the UK and the US said that, from his substantial body of published research, Dr Mohammadi was unlikely to have been working on Iran's nuclear programme, and that his expertise was in another field of physics altogether - quantum mechanics.

There has been much controversy over Iran's nuclear activities.

Tehran says its nuclear programme is for peaceful energy purposes, but the US and other Western nations suspect it of seeking to build nuclear weapons.

On Saturday, Iran said its first atomic power plant in the southern city of Bushehr had begun operations, ahead of a new round of talks with Western powers over the country's nuclear drive.