South Sudan's army advances on rebels in Bentiu and Bor
South Sudan's army is advancing on the key rebel-held centres of Bentiu and Bor, as rebels strengthen defences in Bentiu.
An army spokesman was quoted as saying that government troops were "next to Bentiu" and some 15km (9 miles) away from Bor. Reports say hundreds have fled violence in Bor.
At least 1,000 people have been killed in fighting since 15 December.
Regionally brokered talks to declare a ceasefire have stalled.
People who escaped the violence told AFP news agency that gunmen shot dead fleeing civilians, torched entire villages and looted crops.
A cattle herder who swam across a river to escape said he was fortunate to survive.
At the scene
More than 20,000 South Sudanese are now crammed into a camp meant for 400. The numbers keep growing with more than 2,000 arriving every day.
Conditions are dire - inadequate food, no shelter and hardly any water with only two boreholes serving thousands.
The camp's health centre is overflowing with pregnant women, children and the elderly.
Security is an issue too. There are reports of ethnic fighting here between the Dinka and the Nuer and, as this is a transit centre, authorities cannot separate them yet.
But despite the conditions you see a resilience, a hope that a new future can be carved out of this mess. Many think it lies in a new start, one that recognises their differences but appreciating that South Sudan is bigger than their tribal allegiances.
"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 25km (15 miles) south of Bor.
About 80,000 people had already taken refuge there, AFP reports.
Meanwhile, South Sudan's rebels are strengthening their defences in Bentiu in anticipation of a government offensive to recapture it.
The BBC's Alastair Leithead in the city says rebels brought a tank into the city in the oil-rich area, as the front line moves closer.
However, he says the city was quiet on Friday morning.
On Thursday, heavily armed men looted Bentiu's city centre, while thousands of residents fled to a UN base, a UN official said.
More than 200,000 people have been displaced by the fighting.
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 early on Thursday.
Two signposts summed up how much of a setback the reconciliation between the communities here has suffered. One pointed left and said Dinka, the other pointed right and said Nuer”
It seems the rebels were destroying ammunition as they withdrew 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 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.
More than 2,000 people are fleeing into neighbouring Uganda from South Sudan every day, the BBC's Kasim Kayira reports from northern Uganda.
There are not enough medical and sanitation facilities in the area to cope with the number of arrivals, our correspondent 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.