The UN has launched a "massive emergency operation" to help those affected by ethnic clashes in South Sudan's Jonglei state.Food distribution has begun for 2,000 people, UN spokeswoman Elisabeth Byrs told the BBC.The operation aims to help 50,000 people who have fled recent clashes between rival ethnic groups, she said.The clashes around the town of Pibor are a result of cattle raids that have spiralled out of control.On Thursday South Sudan declared a disaster in Jonglei state.Some 6,000 ethnic Lou Nuer fighters attacked the area around Pibor in recent days, outnumbering South Sudan's army and UN forces.This is the latest round in a cycle of violence which has lasted several months - in one incident last year some 600 Lou Nuer were killed by attackers from the Murle community, the group which fled from Pibor.Several UN agencies are taking part in the aid effort, which will focus on food distribution, providing water and sanitation, and treating the injured, Ms Byrs said."Many of [the displaced] have fled into bush, some of them have walked for 10 days... they are really exhausted," she said.The situation in Pibor was now "calm" and 4,700 people had returned, she said, but added that "houses have been burnt and looted"."Parts of the town have been burnt, our facilities were completely looted, but people are coming back and are not afraid any more, it is stable now," Parthesarathy Rajendran, head of mission for the charity Medecins sans Frontieres (MSF), told AFP news agency after visiting Pibor.Deadly vendettasDozens of people have reportedly been killed in the clashes but the UN could not confirm the figures, saying it was difficult to get an accurate picture of the situation.Cattle vendettas are common in South Sudan, as are other clashes between rival groups. The UN says some 350,000 people were displaced because of intercommunal violence last year.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.