Obama urges peace agreement in South Sudan

Government forces pressed with an offensive to wrest back South Sudan's main oil hub from rebel forces as the White House urged both sides in the deadly conflict to sign an agreement to cease hostilities immediately.

The worst fighting on Friday centred around Bentiu, where forces loyal to former vice president Riek Machar have been holding off the army of President Salva Kiir, leaving the town ransacked and emptied of its civilian population.

South Sudanese rebels rejected a government plan on Wednesday to end a dispute over detainees and unblock peace talks aimed at halting fighting that has killed at least 1,000 people.

President Barack Obama's national security adviser Susan Rice said in a statement it was the responsibility of both leaders to make sure their country was not marred by more violence.

"Machar, in particular, must commit to a cessation of hostilities without precondition," Rice said adding that the US was "disappointed" senior politician detainees in government custody have not yet been released.

"The United States reiterates its call upon President Salva Kiir to release the detainees immediately to the custody of IGAD so that they can participate in the political negotiations."

On Wednesday, the government proposed to shift the peace talks to the United Nations compound in Juba, enabling the 11 detainees to attend the negotiations during the day and return to custody in the evening.

South Sudan's presidential spokesman said the rebels appeared to reject that.

The United Nations, which shelters more than 8,000 people at its Bentiu compound, said its mission in South Sudan had cut off military links with the government and was ready to fend off any attack.

UN deputy spokesman Farhan Haq said the outcome of the battle for Bentiu, capital of Unity state, was "unclear and fluid".

He said there are now more than 60,000 people at UN compounds across South Sudan, half of them in Juba and another 9,000 in Bor, the rebel-held capital of Jonglei state.

In total, there are now probably more than a quarter million people displaced by the fighting, UN peacekeeping chief Herve Ladsous said, after briefing the UN Security Council on the crisis.

No current figures on a death toll were available, Ladsous added, but he estimated it was "very substantially in excess" of the 1,000 deaths the UN reported just after conflict erupted on December 15.

The warring sides met face-to-face for the first time on Tuesday in Addis Ababa in a bid to agree a ceasefire but faced new delays after Kiir refused a rebel demand to release 11 detainees, who were arrested last year over an alleged coup plot.