KHARTOUM — Heavy fighting erupted in the capital of Sudan's embattled Blue Nile state on Tuesday evening, a witness said, a day after the newly installed military governor declared that the security situation had returned to normal.

"One hour ago, I heard gunfire and artillery bombardments in different parts of the city. The shooting lasted about 15 or 20 minutes," a Sudanese journalist in Damazin told AFP, adding that the city's power supply had been cut.

Deadly clashes erupted after midnight last Friday in Damazin, and quickly spread to other areas, as the third major conflict since May unfolded in southern districts just across the border from newly independent South Sudan.

Blue Nile's interim military ruler, Yahia Mohammed Kheir, had told reporters that since the Sudanese armed forces (SAF) drove out rebel troops loyal to elected state governor Malik Agar on Friday, the situation had been "very calm."

He said the electricity, water and hospitals were all working normally and that, except for some clashes in the south of the state, the whole of Blue Nile was stable.

Sudan's deputy information minister Sanaa Hamad confirmed that there had been fighting in Damazin on Tuesday but said it was "accidental."

"A soldier from the SAF fired accidentally, and then the army headquarters in Damazin and the police fired back in response," Hamad told AFP, adding that the fighting had now stopped and the situation was again normal.

The Khartoum government has shown itself increasingly determined to assert its authority within its new borders since the south's secession, moving to disarm troops outside its control.

President Omar al-Bashir said on Sunday that the Sudanese authorities would "crush any military or security violations by the SPLM," the ex-rebel Sudan People's Liberation Movement, which is now the ruling party in the south.

Blue Nile, and nearby South Kordofan, where a similar battle has raged for three months, were both key battlegrounds in the devastating 22-year war between Khartoum and the southern rebels.

They are also both heavily divided along political lines, between supporters of the the northern branch of the SPLM, which the governemnt effectively banned last week, and the ruling National Congress Party.

Sudanese officials, including the newly appointed governor Kheir, have said they have proof that South Sudan is supporting the rebels, an accusation strongly rejected by the authorities in Juba.

On a government-organised trip to Damazin on Monday, journalists were taken to the army headquarters, where tanks and heavy artillery were stationed, and where SPLM officers were introduced, who said they had willingly defected to the government side since the fighting broke out.

In a walled courtyard, scores of captured SPLM troops sat on the floor on the ground awaiting orders, some of them "southern," according to one official.

It was not possible to confirm the claim.