At least 20 people died in a shootout between Sudanese soldiers in a southern town, the military said, warning there was a risk of more clashes as the country divided its forces before the south becomes independent.
Fighting with mortars and heavy machineguns broke out in Malakal on Thursday and again yesterday when part of a military unit refused to redeploy with its weapons to the north - part of a separation of forces before the secession of south Sudan.
An overwhelming majority of people from the oil-producing south voted to split from the north in a referendum in January, according to preliminary results released this week.
The referendum was promised in a 2005 peace deal that ended a decades-long civil war between north and south that also set southern tribe against southern tribe, in internal conflicts that have left deep scars.
Northern and southern leaders still have to finalise how they will share out military hardware and security forces - as well as oil revenues and debts - before the south's departure, expected on July 9. Many fear tensions could re-emerge during the negotiations.
"This morning the number of dead (in Malakal) has risen to 20, and this could change at any moment. Searches are continuing and many are wounded ... Both sides were firing mortars and heavy machineguns," said southern army spokesman Philip Aguer.
The dead included two children and a Sudanese driver for the United Nations' refugee agency UNHCR, officials said yesterday.
Malakal is currently patrolled by a combined military unit made up of the north's Sudan Armed Forces (SAF) and the south's Sudan People's Liberation Army (SPLA), a force the UN said was in the process of splitting up before the south's independence.
"SAF are supposed to go north, SPLA stay in the south," said Aguer.
The situation was complicated by the fact that the SAF unit included many southern soldiers drawn from a militia that fought alongside the north during the civil war.
Aguer said it was those southern soldiers in the SAF unit who resisted the redeployment north and began exchanging fire with other members of the same SAF unit.
"This fighting could happen anywhere where SAF has employed former militia. They are not real soldiers and don't understand the arrangement," he told Reuters.
He said the SPLA set up a buffer zone between the two sides and one SPLA soldier died after being caught in the crossfire.
UNHCR staff held a minute's silence in tribute to driver John James Okwath, 26, who died in hospital on Friday after being shot in the chest, the agency said in a statement.