South Sudan's pro-Machar rebels bolster Bentiu defences
South Sudan's rebels are strengthening their defences in the key city of Bentiu in anticipation of a government offensive to recapture it, a BBC reporter there says.
Alastair Leithead says rebels brought a tank into the city in the oil-rich area, as the front line moves closer.
Heavily armed men looted Bentiu's city centre, while thousands of residents fled to a UN base, a UN official said.
At least 1,000 people have been killed in fighting since 15 December.
Regionally brokered talks to declare a ceasefire have stalled.
More than 200,000 people have been displaced by the fighting.
The army said it was also advancing towards Bor, the capital of Jonglei state.
Bentiu and Bor are the two key centres held by the rebels.
The conflict erupted after President Salva Kiir accused his ex-deputy, Riek Machar, of plotting a coup - a charge he strongly denied.
Mr Machar is backed by army deserters, believed to number at least 10,000, and militias from his Nuer ethnic group.
Mr Kiir comes from South Sudan's largest ethnic group, the Dinka.
Although both leaders have influential backers from the other's community, the conflict has often taken an ethnic dimension.
Our reporter says he heard explosions and saw plumes of smoke coming out of a rebel-held military base on the road to Bentiu.
It seems the rebels are destroying ammunition as they withdraw from the front line, to prevent it from falling into the hands of advancing government troops, he says.
Mr Kiir's forces are believed to be about 25km (16 miles) from Bentiu, capital of Unity state.
The state is rich in oil, the main foreign exchange earner of South Sudan.
Oil production has dropped by 20% since the conflict started.
Civilians from the Nuer community have been streaming into the UN base in recent days, unlike a few weeks ago when the rebel seizure of Bentiu led to Dinkas taking refuge at the base, our correspondent says.
The base is now split into three, with a section for each of the two groups and a third for foreign nationals, he adds.
The hospital in Bentiu was empty, a doctor who worked there said.
"Even the wounded patients ran away," Dr Hassan Mugne said on Twitter.
The UN's Deputy Special Representative in South Sudan, Toby Lanzer, tweeted that rebel forces had looted and largely destroyed shops in Bentiu's market.
There were "virtually no" civilians in the centre of Bentiu, while armed men had been looting shops, as well as aid agency property, he said.
Army spokesman Philip Aguer said that government troops were "next to Bentiu" and some 15 kilometres from Bor, the AFP news agency reports.
People who escaped the violence told an AFP reporter in the area that men with machine guns shot dead fleeing civilians, and torched entire villages and looted crops.
A cattle herder who swam across a river to escape said he was fortunate to survive.
"They [the attackers] had a machine gun raised up on a sandbank, and they fired and fired and fired as we swam," Gabriel Bol told AFP.
"The bullets were hitting the water, but we knew we could not stop or they'd shoot us."
Hundreds of people were fleeing to the town of Minkammen, about 25 kilometres south of Bor.
About 80,000 people had already taken refuge there, AFP reports.
On Wednesday, the International Committee of the Red Cross (ICRC) said the humanitarian situation in South Sudan was dire.
South Sudan is the world's newest state.
It became independent in 2011 after seceding from Sudan.