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 says.Alastair Leithead says rebels rolled 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 December 15. 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 Machar strongly denied.Machar is backed by army deserters, believed to number at least 10,000, and militias from his Nuer ethnic group.President Kiir comes from South Sudan’s largest ethnic group, the Dinka.Although both leaders have influential backers from each of the two groups, the conflict has taken on 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.President Kiir’s forces are believed to be about 25km (16 miles) from Bentiu, the capital of Unity state.The state is rich in oil, South Sudan’s main foreign exchange earner.Oil production has dropped by 20% since the conflict started.Civilians from the Nuer community have been flocking 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 looted shops, as well as aid agency property, he said. The BBC’s Kasim Kayira says Uganda is struggling to cope, as thousands flee South SudanArmy spokesman Philip Aguer said 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, plundered 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 kept on firing as we swam,” Gabriel Bol told AFP."The bullets were hitting the water. They could have hit us if we had stopped,” Bol added.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 seceeded from Khartoum in 2011.