THE United Nations peacekeeping force in South Sudan lacks sufficient troops to protect civilians affected by fighting in an eastern region, the UN envoy said.

Tens of thousands of people have been uprooted by clashes between the army and rebel forces in Jonglei state.

Almost all of the 10,000 residents of Pibor town in Jonglei have fled their homes, aid agencies say, following looting by state security forces.

The head of the UN mission, Hilde Johnson, said she has doubled the number of peacekeeping troops in Pibor.

"We cannot sustain a presence with the logistical capacity that we have, with the problems we have with air transport and by road. So we can not protect civilians in big, big, big numbers," Ms Johnson said.

The UN mission has 6560 troops to cover a country the size of France with barely 300km of paved roads.

Seasonal rains have turned the region, where the government hopes to drill for oil with France's Total, into a swamp, severing road access across the area.