The Sudanese army and forces aligned to South Sudan clashed on Friday in a flashpoint state in Sudan that lies on the border with the newly independent south, officials on both sides said.
Sudan's Blue Nile state is home to many supporters of the south's dominant Sudan People's Liberation Movement (SPLM). Khartoum has previously threatened to disarm southern-aligned fighters in Blue Nile.
Malik Agar, governor of Blue Nile and member of the northern branch of the SPLM, told Sudan's state news agency SUNA that clashes erupted between the SPLM-aligned forces at the entrance to the Blue Nile state capital al-Damazin and Sudanese army forces during the night of Thursday to Friday.
"Clashes quickly spread to all areas where (forces of the SPLM) were stationed," SUNA quoted him as saying, adding that no casualties had been reported.
South Sudan split from the rest of the country in July after a referendum on secession, part of the 2005 peace deal that ended decades of conflict between north and south. The separation was relatively smooth but tensions simmer.
Under the 2005 deal, residents of Blue Nile state and South Kordofan, another state where Sudan's army and southern-aligned forces have clashed, were offered "popular consultations" to determine ties to Khartoum. These have not been completed.
Yasir Arman, the secretary-general of the SPLM's northern branch, said in a statement that the Sudanese army attacked the residence of Al-Jundi Suleiman, commander of the joint integrated units in Blue Nile, and then other areas.
"The offensive was later intensified to include all SPLA (Sudan People's Liberation Army) positions," he said, adding that the Sudanese army had in the past few days moved armaments including tanks into the area.
Sudanese army spokesman Al-Sowarmy Khaled Saad told Reuters that SPLM forces had attacked the Sudanese army late on Thursday in and around al-Damazin but the army was now in control.
In a statement carried by SUNA, the Sudanese government said it had issued orders for the SPLM to hand over those involved and said it would arrest those who refused.
It blamed units loyal to the SPLM of launching an attack on soldiers of the Sudanese army who were also part of the integrated forces in Blue Nile.
Similar clashes and mutual accusations about who was to blame have led to an escalation in violence in South Kordofan, another state in the north that is on the southern border.
One observer who has worked in Blue Nile and the south said southern-aligned forces in Blue Nile were indigenous to the region and, unlike southern army units that had to move south after secession, these units could not be moved out.