Thousands of youths from a South Sudanese ethnic group which attacked a rival community, reportedly killing at least 150 people, have been repelled by government troops, the UN says.The UN's humanitarian co-ordinator in the region, Lise Grande, says 6,000 members of the Lou Nuer ethnic group have left the besieged town of Pibor.The clashes took place between the Lou Nuer and their rivals, the Murle.The fighting follows long-running disputes over cattle raids.As well as those killed, tens of thousands have been displaced in the violence, according to the South Sudan government."Pibor is under the full control of the government, and the Lou Nuer have been ordered to return to their homes, and they are starting to do so," Information Minister Barnaba Marial Benjamin said.Ms Grande said a decisive event took place on Monday: government troops backed by UN forces repelled an attack on Pibor in Jonglei state by the Lou Nuer ethnic group; shortly after that, she said, the Lou Nuer began to leave the area.The Lou Nuer had launched an offensive on their rivals from the Murle ethnic group last week, accusing them of stealing cattle.Ms Grande said damage to Pibor was limited.However, she said the humanitarian situation for the tens of thousands of people who fled the violence was grim, with the UN and other agencies now organising an emergency programme to help them.Ms Grande said the government was beginning to deploy 3,000 extra soldiers and 800 police officers to the area.She also said the Lou Nuer took a lot of cattle with them.Last August, it was the Murle who attacked them and raided their herds.Cattle vendettas are common in South Sudan, as are other ethnic and tribal clashes: the UN says some 350,000 people were displaced because of intercommunal violence last year, says the BBC's Barbara Plett at the United Nations.This presents a major challenge to the government of the newly independent state, which also faces cross-border tensions with its northern neighbour Sudan.The clashes began as cattle raids, but have spiralled out of control.Town burntSouth Sudan is one of the world's poorest regions - it gained independence from Sudan in July 2011 and has hardly any roads, railways, schools or clinics following two decades of conflict, which have left it awash with weapons.South Sudan's President Salva Kiir had called on the Lou Nuer to stop their advance and return to their traditional areas.The Lou Nuer fighters arrived in Pibor on Saturday after marching through Jonglei state in recent weeks, setting fire to homes and seizing livestock.The entire town of Lukangol was burnt to the ground last week. About 20,000 civilians managed to flee before the attack, but dozens were killed on both sides.The governor of Jonglei state and the vice-president of South Sudan have been trying to mediate between the rival ethnic groups.