Columns of heavily armed South Sudanese gunmen in a tribal militia are fighting their way towards a rival community in the eastern Jonglei state, a local official said Friday.

Lou Nuer and Dinka gunmen from northern Jonglei are reported to be marching south towards Pibor, an area of their rivals, the Murle.

"We are worried that a lot of people will die, especially women and children," said Pibor County Commissioner Joshua Konyi, speaking to AFP via a crackling satellite phone from the town.

He said civilians further out in the swampy bush were reporting that Murle civilians were fleeing ahead of rampaging gangs of Lou Nuer gunmen armed with semi-automatic rifles, machine guns and rocket-propelled grenades.

"I'm worried because the Lou Nuer youth are there in huge numbers," he said, adding that some reports estimated the militia force to number several thousand.

The latest round of fierce fighting broke out a week ago in Pibor county in conflict-wracked Jonglei, between government troops, rebels and ethnic militia.

The United Nations peacekeeping mission in South Sudan (UNMISS) is "deeply concerned about reports of a major mobilisation of armed youth (and) reported clashes," it said in a statement late Thursday.

The latest clashes follows bitter fighting in May, when soldiers and other gunmen looted UN and aid agency stores in Pibor, including a key hospital.

US State Department officials said Thursday they were "deeply disturbed by mounting reports of abuse of civilians, including targeted killings, rape, beatings, and the looting and destruction of homes and humanitarian facilities."

South Sudan's rebel-turned-official army has also been fighting in the region to crush a rebellion since April 2011 led by David Yau Yau, who comes from the Murle people.

Government officials in Lou Nuer areas in northern Jonglei denied young men had set off to fight, but past clashes have followed a similar pattern.

In December 2011, some 8,000 Lou Nuer marched south killing and looting in what they said were reprisals for earlier attacks and cattle raids by Murle fighters.

The UN later estimated over 600 people were massacred, although local officials reported the figure to have been far higher.

Rights groups accuse all sides of abusing and raping civilians.

UN peacekeepers are based in Pibor and "are patrolling in the town", but are not deployed in frontline areas, Konyi said.