"We have deployed a battalion-sized force in Pibor to support the government to protect civilians," Lise Grande, the UN humanitarian coordinator for South Sudan, told AFP Saturday, adding that the provision of food aid was an "urgent step."
She said the peacekeeping reinforcement had been deployed within the past three days, with a company of blue helmets being the latest addition on Friday. "We are extremely concerned about the massing of the Lou Nuer youth," in that part of the newborn nation's troubled Jonglei state, Grande said.
An estimated 500 members of the Lou Nuer tribe -- among 6,000 armed youth who have been marching towards Pibor -- were already on the town's outskirts, the United Nations warned on Friday.
Thousands from the rival Murle tribe began fleeing towards Pibor several days ago when the Lou Nuer raided the town of Lukangol, but many have now also escaped from Pibor, the UN said.
A peace mediator in Jonglei said gun-wielding Lou Nuer youths had wounded more than 60 people, killing at least one, in attacks over the past four days.
"We are waiting (to see) if they will launch an attack again, or they will not. But this morning it was calm," said Gatwech Koak Nyuon, whose church-affiliated Nuer Peace Council says it has worked since 2004 to bring peace to various South Sudanese groups, not only the Nuer.
To assist those left in the town, the UN's World Food Programme flew in a helicopter with food on Saturday, Grande said, after plans for an airlift on Friday were suspended following reports that security was deteriorating.
"As of yesterday there was no food in Pibor, so this was an urgent step," Grande said. "We're going to try to assist several thousand people for up to two weeks," she added. "But of course that depends on security conditions."
Grande said about 100 "highly vulnerable" civilians -- including sick and elderly -- were stuck in the town centre. "We're looking to evacuate them," she said. "It's a top priority for us."
On Friday the UN said two armoured personnel carriers had managed to reach Pibor but the roads were virtually impassable.
A group calling itself the Nuer Youth White Army issued a statement on December 26 vowing to "wipe out the entire Murle tribe...as the only solution to guarantee long-term security of Nuer's cattle."
The group accuse the Murle of raiding Nuer cattle and killing members of their tribe since 2005, when a peace agreement ended two decades of civil war and led to South Sudan's independence this year.
Neither the UN nor South Sudan's former rebel army the SPLA have protected the Nuer, the group alleged.
"We the Nuer Youth have decided to fight the Murle, SPLA and the UN," it said. The UN had raised the alarm in September over cattle raids that had already left around 1,000 people dead since June, calling it a crisis that threatened to engulf the fledgling nation.
The raids involved "army-like" movements of people with new weapons and satellite phones, the UN said at the time.
South Sudan's army spokesman has alleged that the assailants were armed and supported by Khartoum, which has in turn accused Juba of supporting rebels within its territory.
The South separated from Sudan in July after an overwhelming vote for independence following decades of conflict that left some two million people dead.