About 750 people have died this month in clashes in South Sudan over cattle-rustling, the United Nations says.
The UN said the fighting was sparked by livestock raids between the Murle and Nuer ethnic groups in Jonglei state.
The delivery of aid to the region has been disrupted, with fighters attacking a food convoy on Tuesday.
The UN says security is deteriorating in South Sudan as ex-rebel fighters stage violent protests over non-payment of benefits for the past five months.
The co-ordinated demonstrations by former fighters in the south's military, the Sudan People's Liberation Army, have paralysed some towns in the Central and Eastern Equatoria regions.
Correspondents say a visit by South Sudan's President Salva Kiir has helped to settle parts of the region but areas in the east are reported to remain tense.
This month's fighting has broken out in the flashpoint of South Sudan where the 1983-2005 civil war erupted.
"We're seeing reports of perhaps up to 750 dead, that seem to have been generated by cattle rustling," Geoff Wordley, assistant representative of the UN High Commissioner for Refugees (UNHCR) in South Sudan, told the BBC.
"The situation now appears to have calmed but during the disturbances at least one World Food Programme convoy was attacked and looted."
The BBC's Peter Martell in the South Sudanese capital Juba says the clashes are scaring off thousands of refugees who had been considering returning home after years in exile.
It follows separate fighting last month in the capital of Upper Nile State between northern and southern troops in a joint unit.
The conflict between north and south ended with the loss of an estimated 1.5 million lives and the setting up of an autonomous secular government in the south.