"We fear there may be more dead to find," Yol said said from Bor, the capital of Jonglei.

"The situation in Akobo town is tense, and those fleeing the fighting are continuing to arrive into the town."

'Political agitators'

Salva Kiir, the president of the autonomous government in the south, has blamed political agitators who he said want to show that the south cannot run itself ahead of a promised 2011 southern referendum on separation from northern Sudan.

Disputes, many sparked by cattle rustling, have been exacerbated by a ready supply of arms left over from the two-decade civil war between north and south Sudan, and political disaffection over the slow pace of development in the region.

South Sudanese and UN officials had hoped the recent onset of the region's rainy season would reduce the violence, as heavy downpours restricted access to remote villages.

 "This year there has not been enough rain to lessen the movements," Hussein May Nyuot, the deputy governor of Jonglei, said on Monday.

Jonglei state was one of the areas hardest hit in Sudan's two-decade-long civil war, which ended in 2005 with a power-sharing deal between the predominantly Muslim north and the largely Christian an animist south.

There are large numbers of arms in the state and there are frequent clashes between rival groups. Attempts at disarmament have left some regions at risk of attack from their still heavy-armed neighbours.

Under the deal, the south began a six-year transitional period of regional autonomy until a referendum on self-determination in 2011.