Thousands of people are said to be 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 United Nations base in Bentiu, a BBC reporter there says.
Government troops are believed to be about 25km from the capital of Unity state on Thursday, an area rich in oil and the main foreign exchange earner of South Sudan.
Bentiu and Bor, the capital of Jonglei state, are the main centres under rebel control.
Meanwhile, ceasefire talks between the sides appear to be deadlocked over the government's imprisonment of 11 alleged coup plotters.
At least 1000 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.
Many foreign governments have evacuated their nationals, while many South Sudanese are crossing by land into neighbouring states.
The conflict broke out in mid-December last year when President Salva Kiir accused his former 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, regional mediators Seyoum Mesfin and Lazurus Sumbeiywo flew out of South Sudan's capital, Juba, after talks with Mr Kiir. However, they failed to break the deadlock over the fate of the detainees - a key obstacle to the two sides negotiating a ceasefire.
Mr Kiir offered to free the detainees to take part in the regionally-brokered talks, if the discussions were moved from Ethiopia to Juba. However, he said they would have to return to their places of detention at night. That offer was rejected by Mr Machar's allies.