Juba - South Sudan's deputy defence chief has said neither his troops nor United Nations peacekeepers are able to protect civilians in conflict-wracked Jonglei, where thousands of rival ethnic militiamen are fighting.
Video footage from eastern South Sudan's Jonglei shot by UN officials and seen by AFP show columns of heavily armed fighters from the Lou Nuer people marching past, watched on by a small force of government troops and UN peacekeepers.
"Much as we believe in the ideals of the responsibility to protect, our mandate as the government and the mandate of the UN cannot match with resources that are there," South Sudan's deputy minister of defence Majak D'Agoot told AFP late on Tuesday.
The video was shot on Sunday in the village of Manyabol in Pibor County, when the UN went to support the pickup of some 200 wounded fighters - casualties from almost two weeks of fighting in the latest round of reprisal attacks sparked by age-old ethnic rivalry and cattle raiding.
The video shows fighters apparently returning towards their homelands, some leading stolen cattle.
The numbers of fighters suggest attacks on a scale comparable to those of December 2011, when some 8 000 Lou Nuer marched on Pibor, home town of their long-term rivals, the Murle people.
The UN later estimated more than 600 people were massacred in those attacks, although local officials reported the figure to have been far higher, while killings continued in a series of reprisal attacks.
D'Agoot said that in Manyabol the army had only one company, alongside a handful of UN peacekeepers, and that they were vastly outnumbered by as many as 7 000 militia gunmen.
Taking action in those circumstances would have been "suicidal", he said.
Hilde Johnson, head of the UN mission in South Sudan (UNmiss), said she had not seen the video her force had shot, but said that peacekeepers had sighted Lou Nuer forces and were "verifying that they were moving north on their return home".
Tit-for-tat cattle raids and reprisal killings are common in this severely under-developed state, awash with guns left over from almost two decades of civil war.
But recent attacks are on larger scale, with organised and well armed forces fighting.
South Sudan's rebel-turned-official army has also been fighting in the region to crush a rebellion led by David Yau Yau, who comes from the Murle people, since 2010.