Thousands of people are fleeing the South Sudanese city of Bentiu amid fears of a government offensive to recapture the oil-rich area from rebels.
Many people are taking refuge in a UN base in Bentiu, says Alastair Leithead.
Meanwhile, ceasefire talks between the two sides appear to be deadlocked over the government’s imprisonment of 11 alleged coup plotters.
At least 1,000 people have been killed in the conflict.
Nearly 200,000 people have been forced from their homes in the fighting, which has seen ethnic violence between the Dinka and Nuer communities.
The conflict broke out in mid-December when President Salva Kiir accused his ex-deputy, Riek Machar, of plotting a coup.
Mr Machar denied the allegation, and called for the unconditional release of 11 of his allies who were detained over the alleged plot.
On Wednesday, Mr Kiir offered to free them to take part in the regionally-brokered talks, if the discussions were moved from Ethiopia to the South Sudan capital, Juba.
However, he said they would have to return to their places of detention at night.
This offer was immediately rejected by Mr Machar’s allies.