JUBA - South Sudan government forces were moving into the flashpoint town of Pibor on Sunday to tackle escalating tribal violence that has forced thousands of people to flee and put the United Nations on high alert.

Doctors without Borders, the only organisation providing health care in the area, said its staff had mostly fled into the bush, while there were reports of damage on the outskirts of town, including huts being torched and cattle raids.

The government and the UN -- which has warned the violence could lead to a "major tragedy" -- were beefing up their forces in Pibor as a column of 6,000 armed youths from the Lou Nuer tribe marched on the town pursuing a rival tribe.

Lise Grande, the UN humanitarian coordinator for South Sudan, told AFP that the number of government forces heading to Pibor in the troubled Jonglei state was estimated at 3,000 troops and 800 police.

"The government is firmly in charge of the situation and is taking very bold steps. They are deploying large numbers of infantry soldiers and police who are already moving towards Pibor," she said.

"We remain on high alert and are very concerned that civilians may be at risk," she said, adding that the United Nations was evacuating the most vulnerable civilians from the town and that the situation remained "very fluid".

"Our understanding is that there has been damage to the periphery of the town, including the torching of tukuls (thatched huts) and that the compound of one humanitarian organisation has been taken over.

By Sunday evening, most of the tribal youths appeared to have entered the town, but have caused only limited damage to property, a humanitarian source in Juba told AFP, asking not to be named. Parthesarathy Rajendran, head of mission at Doctors without Borders (Medecins sans Frontieres - MSF) said staff at its hospital in Pibor town and two outreach clinics in the area have for the most part fled into the bush.

"What we are hearing is that our clinic has been damaged and a lot of things looted," he told AFP.

"Since the start of the fighting one week before, the whole population has started displacing and running into the bush," he said, adding that MSF is the sole provider of health care in Pibor county.

The Lou Nuer youths, several hundred of whom had arrived on the outskirts of Pibor on Friday, were pursuing members of the Murle tribe who began fleeing towards Pibor several days ago after the Lou Nuer raided the town of Lukangol.

The MSF spokesman said Lukangol had been reduced to ashes.

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 cattle".

The group accuses 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 United Nations nor South Sudan's former rebel army the SPLA have protected the Nuer, the group claimed.