At least 13 people, including two children, have been killed in clashes between soldiers in the volatile south Sudan town of Malakal, doctors say.
Battles broke out on Thursday between rival northern troops, some of whom want to stay in the south. Malakal has previously seen north-south clashes.
The fighting comes as Southern Sudan is waiting for confirmation of the result of its independence referendum.
Provisional results say 99% of voters opted to secede from the north.
The vast majority of casualties seen in the hospital are civilians, caught in the battle which officials say has included heavy weapons such as mortars.
But more are feared dead in the areas of heaviest fighting, which doctors have not yet been able to access.
The fighting began when southerners who joined the northern army did not want to move, Upper Nile state spokesman Bartholomew Pakwan Abwol told the Reuters news agency.
"They think they will have no rights in the north," he said.
Another of the dead was a United Nations driver caught in crossfire, a UN spokesman said.
The southern army, the SPLA, has not become involved in the fighting.
It has previously clashed with northern militia in the town on the River Nile, leaving hundreds dead in 2006 and 2009.
It is still seen as one of the potential flashpoints along the north-south border.
Southern Sudan is set to become the world's newest nation on 9 July 2011.
Its referendum was part of a deal to end decades of conflicts between north and south, driven by religious and ethnic divides.