Renegade militia fighters clashed with south Sudan's army for a second time in three weeks, both sides said, reigniting fears for the stability of the oil-producing region in the countdown to its independence.
Rebel leader George Athor said nearly 100 people died in the latest fighting on Sunday in the south's Jonglei state - where France's Total oil giant is due to start exploring.
The south's army said fighting continued on Monday but said the rebels’ death count was exaggerated.
The violence has cast a shadow over mass celebrations after southerners overwhelmingly voted to declare independence from the north in a referendum in January.
The referendum was promised in a 2005 peace deal that ended decades of civil war with the north, a conflict that also saw fighting between rival southern militias.
Athor accused the southern Sudan People's Liberation Army (SPLA) of starting last month's fighting and breaking the terms of a ceasefire agreed in January.
"They attacked us early in the morning on Sunday. We dispersed the SPLA forces and we captured a big number of arms. Also we managed to kill 86 soldiers. We lost 12 of our comrades," he said.
Athor insisted he was ready to return to negotiations with the southern leadership.
"I am really worried because the new country will be like a baby born dead. If you start with a guerrilla force fighting the government, I don't see any development that can happen."
Athor was a senior member of the rebel southern army during the civil war.
He stood for the governorship of Jonglei as an independent against Kuol Manyang, the incumbent for the south's ruling Sudan People's Liberation Movement (SPLM), in last year's elections.
Athor took to the bush after losing, accusing the SPLM of fraud. Today, he said he was fighting to bring democracy to the south.
The south has regularly accused north Sudan of arming his forces to destabilise the region, an allegation denied by Khartoum and Athor.
Jonglei's governor Kuol Manyang said the army had sent more troops to the territory bordering Ethiopia after Athor was accused of carrying out a massacre of more than 200 people in Jonglei's Fangak area last month.
Southern army spokesman Philip Aguer said about 40 if its soldiers died in the fighting, and had no estimate of Athor's casualties. "I don't know why he is doubling the number. Why is he so proud of killing people?" he asked.
Reuters