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, a BBC reporter there says.
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.
Government troops are believed to be about 25 kilometres (16 miles) from Bentiu, capital of the oil-rich Unity state, our correspondent says.
Unity state is rich in oil, the main foreign exchange earner of South Sudan.
Oil production has dropped by 20% since the conflict started.
Bentiu and Bor, the capital of Jonglei state, are the two main centres under rebel control.
Fighting was continuing around Bor, as government forces tried to recapture it, army spokesman Philip Aguer said, AFP news agency reports.
It says its reporter reached the town of Minkammen, 25 kilometres south of Bor, and the area was flooded with fleeing civilians.
Heavy artillery fire could also be heard in the distance, AFP reports.
South Sudan is the world’s newest state.Â It became independent in 2011 after seceding from Sudan.