Up to 50,000 flee South Sudan tribal turmoil: U.N. CAIRO (Reuters) - Up to 50,000 people have fled tribal violence in a remote border area of South Sudan, the United Nations said Monday, in the latest episode of upheaval to hit the new African nation. South Sudan became independent in July last year under a 2005 peace deal with Khartoum to end decades of civil war. But the new nation is struggling to build state institutions and stop rebel and tribal bloodshed that has killed thousands. Monday, some 6,000 armed members of the Lou Nuer tribe attacked the remote town of Pibor in Jonglei state bordering north Sudan after days of clashes with the rival Murle tribe, U.N. sources said. Lise Grande, U.N. humanitarian coordinator for South Sudan, said tens of thousand of civilians had fled Pibor and other nearby towns to escape the violence. We are worried about their conditions. They are without water, shelter and food. They are hiding in the bush. I think it is between 20,000 and 50,000. This is an estimate only," she told Reuters. She had no information about casualties. South Sudan's armed forces are sending reinforcements to Pibor, army spokesman Philip Arguer said. They attacked the town this morning. Civilians were evacuated from Pibor three days ago," he said, without giving further details. U.N. sources said around 3,000 soldiers and 800 policeman were on their way to Pibor. Cattle raids, they said, had sparked the latest violence. Reporting by Ulf Laessing; Writing by Tom Pfeiffer; Editing by Mark Heinrich