The UN has defended the role of its peacekeepers and South Sudan government soldiers after deadly ethnic clashes near Pibor town in Jonglei state.Lise Grande, the UN's humanitarian co-ordinator in the region, told the BBC the town had been successfully defended from some 6,000 Lou Nuer fighters.Tens of thousands of town residents, members of the rival Murle community, have sought safety in the bush.Ms Grande said "a major humanitarian operation" was now a priority.This is the latest round in a cycle of violence which has lasted several months - in one incident last year some 600 ethnic Lou Nuer were killed.The clashes began as cattle raids but have spiralled out of control.John Boloch of South Sudan's Peace and Reconciliation Commission said at least 150 people, mostly women and children, had been killed since the Lou Nuer fighters arrived in the vicinity of Pibor on Saturday.Ms Grande said she had visited Pibor on Tuesday and that "hundreds if not thousands" of people had begun to return to the town after about a week in the bush."The humanitarian situation is pretty grim," she told the BBC's Network Africa programme."They've been without food, they've been without water, without shelter."She said the "the main body of the Lou Nuer youth" who had been outside Pibor had started move in a north-easterly direction and were now 15km (about 10 miles) from the town.They had a large number of cattle with them, the envoy said, AFP news agency reports.'Hunted and killed'She rejected criticism of the UN and the South Sudan army - the SPLA - of not having done enough to help civilians, many of whom reportedly faced attack when they fled the town."What the United Nations mission has been doing is helping the government to defend the town, we've been rescuing civilians, we've been evacuating civilians and we've been helping to deter violence. The UN has done its job," she said.An attack on the town's southern flank had been repulsed after the SPLA, backed by UN armoured personnel carriers, had fired at the Lou Nuer, she said.Besides the looting of a Medecins Sans Frontieres clinic, the town had not suffered much damage and the government was beginning to deploy 3,000 extra soldiers and 800 police officers to the area, Ms Grande said.Earlier, Mr Boloch, from the Murle community, said people who had fled Pibor had since been hunted down and killed near the River Kengen, south-east of the town."Children and women were massacred in that area… on the 2nd [of January], up to the 3rd," he told Sudan Catholic Radio News.He accused local politicians of exacerbating the long-standing rivalries for their own ends and also asked why UN peacekeepers and the army were protecting government buildings in Pibor, rather than people.Another woman was in tears as she told the BBC that her mother had called to say that 20 family members had been shot dead on Monday - all women and children."They had fled Pibor before the fighters reached there. They were hiding about three hours' walk away by the River Kengen," she said.There are also reports that many people may have drowned in a river as they fled the attacks.Town burntCattle 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, reports 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.South 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.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.