At least 16 people have been killed after rebels attacked south Sudan's army, shattering a ceasefire, officials say.
Forces loyal to George Athor blew up two army trucks near the town of Fangak in Jonglei state, a southern military spokesman said.
Mr Athor took up arms last year, alleging fraud in state elections, but signed a ceasefire last month.
The clashes come as Southern Sudan prepares to secede from the north.
Some 99% of southerners voted for independence in last month's referendum, according to official results announced this week.
Southern army spokesman Philip Aguer said four soldiers and 12 rebels were killed but he feared that many more people had died.
"We are still waiting for full details of the casualties," he said.
When Mr Athor took up arms last April, the south accused him of being used by the north to stir up trouble and derail the referendum - charges denied by northern officials.
He agreed to the ceasefire deal with the south's Sudan People's Liberation Army (SPLA) days before the referendum vote began - although he did not attend the signing ceremony in in person.
"Indeed, we were surprised by the attack, because the SPLA were busy transporting food to the sites of assembly, where the men of Athor are due to gather under the terms of the agreement," Col Aguer told AFP news agency.
The BBC's Peter Martell in the southern capital, Juba, says the fighting is another sign of the challenges the south faces in bringing its people together and improving security.
The week-long referendum vote itself passed off peacefully, but tension remains high in parts of the oil-rich area which straddles the north and south. Fifty people were killed over the weekend in fighting in Southern Sudan's Upper Nile state.
Sudan's President Omar al-Bashir has promised to accept the outcome of the referendum.
On Wednesday, Sudan's UN ambassador hinted that the International Criminal Court arrest warrant for Mr Bashir should be withdrawn as a "reward" for him accepting the south's independence.
Mr Bashir is accused of links to war crimes in a separate conflict in the western region of Darfur.
Southern Sudan would become the world's newest independent state on 9 July.
The great divide across Sudan is visible even from space, as this Nasa satellite image shows. The northern states are a blanket of desert, broken only by the fertile Nile corridor. Southern Sudan is covered by green swathes of grassland, swamps and tropical forest.