by Rashida Adjani and Biodun Iginla, BBC News

Many Blue Nile residents fought for the SPLM during the long north-south conflict

Fighting has broken out in Sudan's Blue Nile state - the third border area to see clashes since South Sudan's independence in July.

The party of Blue Nile Governor Malik Agar said soldiers had attacked his official residence. The army blamed the fighting on Mr Agar's forces.

He heads the opposition SPLM-North party and is an ex-commander of the rebels who now govern South Sudan.

Some 200,000 have fled their homes in neighbouring South Kordofan.

The government has denied charges its forces have carried out ethnic cleansing in the area against groups seen as being pro-south.

On Tuesday, Sudan's government lodged a complaint with the UN Security Council, accusing South Sudan governing party of backing rebels in South Kordofan, which the Sudan People's Liberation Movement (SPLM) denies.

There has also been fighting in Abyei, which is claimed by both sides.

The BBC's James Copnall in Juba says this is the first major outbreak of fighting in Blue Nile since the north-south conflict ended in 2005.

But he says the region was always a high risk state because it borders South Sudan and is split between supporters of President Omar al-Bashir's government and the SPLM.

Sudan said Mr Agar's forces launched attacks on police stations and government buildings in the state capital, Damazin.

"The Sudanese Armed Forces responded to this attack and has now driven the rebels away. The government now controls Damazin and the area around it," government spokesperson Rabbie Abdelatti told the BBC.

A local resident said government tanks were patrolling the city and forces loyal to Mr Agar have withdrawn from the city.

Many residents have fled Damazin because they fear the violence could escalate, the resident said.

He and a UN source told the BBC fighting flared up on Thursday night and shooting could still be heard in Damazin on Friday.

The SPLM-North, which says it has split from the south's ruling party, says the army moved into strategic positions of Damazin with 12 tanks and 40 other armoured vehicles backed up by militia fighters.

It accused the government of declaring a surprise ceasefire in South Kordofan last week to prepare the ground for an attack in Blue Nile.

The SPLM led the decades-long struggle which led to South Sudan's independence.

When a peace deal was signed in 2005 to pave the way for the South's independence, the SPLM and Khartoum agreed Blue Nile and South Kordofan would have "popular consultations" about their future.

But these did not take place properly, leading to an increase in tensions and now clashes, our reporter says.

Mr Agar recently warned Khartoum of the dangers of taking on rebels in South Kordofan, Blue Nile and Darfur because they were likely to co-ordinate their efforts.

Opposition groups in the three areas accuse Khartoum of ethnic cleansing and failing to give them a fair share of the national wealth.

The International Criminal Court (ICC) has issued arrest warrants for Mr Bashir and the South Kordofan governor over the conflict in Darfur, accusing them of genocide and war crimes.

Mr Bashir denies the allegation and says the ICC is controlled by Western powers hostile to Sudan.

Last month, a UN report said both government and rebel forces may have committed war crimes in South Kordofan, but it said the army's actions were "especially egregious" - referring to summary executions, aerial bombardments and the shelling of neighbourhoods.

When South Sudan split from the north on 9 July, the new country's leader, Salva Kiir, said he would work with Mr Bashir to ensure the rights of former southern rebels in the north were respected.