Heavy gunfire broke out on Tuesday in the capital of Sudan's Blue Nile border state where government soldiers have been fighting armed opposition groups, a Reuters correspondent said.

Tensions have mounted in states along Sudan's poorly-defined border with South Sudan since the south declared independence in July.

Thousands fled after fighting erupted last week in Blue Nile -- the third Sudanese border area hit by violence this year between the army and forces linked to South Sudan's ruling Sudan People's Liberation Movement (SPLM).

A Reuters journalist in the state capital Damazin said he heard intense gunfire lasting several minutes late on Tuesday. The power supply was interrupted after the incident in parts of the city, he added.

The Sudanese army said a government soldier had accidentally fired his gun outside Damazin and other soldiers inside the city responded by shooting their weapons, state news agency SUNA reported.

"The situation is now quiet. There is no attack by the enemy against the city," an army spokesman told SUNA. No one was immediately available for comment from the armed groups allied to the opposition SPLM-N, the former northern wing of the South Sudan's ruling SPLM.

Blue Nile and Southern Kordofan -- both states on Sudan's side of the border -- and the disputed Abyei area, saw heavy fighting during decades of civil war between the Khartoum government and South Sudan. Fresh clashes have broken out in all three this year.

They are all still home to tens of thousands of people from ethnic groups that sided with the south during the civil war.

Khartoum has accused people from those groups of trying to spread chaos along the border, backed by South Sudan's government -- a charge denied by South Sudan.

Rights groups have accused Khartoum of trying to stamp out remaining opposition on its side of the border and provoking the violence by trying to disarm south-linked groups.

Tuesday's shooting came a day after the new Blue Nile Military Governor Yahia Mohammed Kheir said life in Damazin had returned to normal and residents had started to return.

He told reporters in the state capital on Monday fighting was continuing south of Damazin where the army was fighting groups allied to the SPLM.

"Fighting is going on 30 km (19 miles) to the south but in the west and north and east (of Blue Nile state) the situation is very calm," he said.

On Friday, Sudan's President Omar Hassan al-Bashir appointed Kheir as temporary military ruler after firing the elected governor Malik Agar, from the opposition SPLM-N, the northern wing of the south's dominant SPLM.

When asked when the army would end military operations, Kheir said: "We would like to end military operations today. We are now clearing areas where SPLA fighters are still present."

Officials told reporters 12 soldiers, six policemen and three citizens had been killed in fighting in Damazin since last week.

On Tuesday, Mandour al-Mahdi, a senior official in Sudan's ruling National Congress Party (NCP), said the SPLM would have to stop its political activities in the north as it was not properly registered.

SPLM-N Secretary-General Yasir Arman said in statement authorities were closing its offices throughout Sudan.

South Sudanese voters overwhelmingly chose to declare independence from the north in a 2005 referendum, a vote promised in a 2005 peace deal that ended the north-south civil war fought over oil, religion, ideology and ethnicity.