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.
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 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, regional mediators Seyoum Mesfin and Lazurus Sumbeiywo flew out of South Sudan's capital, Juba, after talks with Mr Kiir.
'Heavy artillery fire'
However, they failed to break the deadlock over the fate of the detainees, a key obstacle to the two sides negotiating a ceasefire, Juba-based journalist Mading Ngor told the BBC's Focus on Africa.
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.
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 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.
The International Committee of the Red Cross (ICRC) said the humanitarian situation in South Sudan is dire.
"South Sudan is facing a serious crisis that comes on top of a situation that was already difficult," ICRC president Peter Maurer said, at the end of a three-day visit to the country.
"It is unquestionable that the needs are dire, but their full scope is unknown," Mr Maurer added in a statement.
South Sudan is the world's newest state.
It became independent in 2011 after seceding from Sudan.
Mr Kiir comes from the largest ethnic group, the Dinka, while Mr Machar is a Nuer.
Both leaders have influential backers in the other group, even though the conflict has taken an ethnic dimension.
Mabior Garang, a Dinka, is a key member of the rebel delegation in talks in Ethiopia. He is the son of veteran southern leader John Garang, who died in 2005.