JUBA: South Sudan's government said Friday it was mobilizing thousands of additional troops as it battled to recapture two rebel-held cities, although regional mediators insisted they were still hopeful for a ceasefire.

Heavy fighting continued to rage around Bentiu, where forces loyal to former vice president Riek Machar have been holding off the army of President Salva Kiir, leaving the northern oil hub ransacked and emptied of its civilian population.

UN deputy spokesman Farhan Haq said the outcome of the battle for Bentiu, capital of Unity state, was "unclear and fluid", and a resident contacted by AFP confirmed that heavy battles were raging close to the outskirts of the city.

Fierce battles have also continued around Bor, another rebel-held town in the centre that has already changed hands three times since the conflict began nearly a month ago.

The United Nations meanwhile said it believed that "very substantially in excess" of 1,000 people have been killed in the fighting, and that nearly a quarter of a million people have fled their homes -- many of them fleeing a wave of ethnic violence between Kiir's majority Dinka tribe and Machar's Nuer.

The United States, which was instrumental in helping South Sudan win independence from Khartoum in 2011, also said it feared the country risked imploding and urged the two sides to immediately agree to a truce.

In the capital Juba, government allies from several regions told reporters they were in the process of calling up thousands of former soldiers to beef up the government army -- a sign the fighting could still escalate.

"We have to mobilise all SPLA soldiers, all former soldiers who were in the Sudanese army," said Clement Wani Konga, governor of Central Equatoria State, adding 3,000 extra troops had been found in his region alone and a further 12,000 were expected to soon be armed and ready.

But in the Ethiopian capital Addis Ababa, where the East African regional bloc IGAD is trying to broker a truce, the chief mediator told AFP he was still optimistic.

"If you ask me on the possibilities of signing, I am very optimistic... because we have now come a long way in establishing understanding between the parties," Seyoum Mesfin said, adding that he expected a ceasefire in "the shortest possible time".

A main stumbling block in the talks has been the release of 11 of Machar's allies who were detained by the government after the unrest began.

Kiir has accused Machar of attempting a coup, and says the detainees will not be released without a legal process. Seyoum said mediators discussed other options, including transferring them to IGAD or United Nations custody or releasing them on bail.

"He's considering all the options," Seyoum said of Kiir, insisting that those jailed have a crucial role to play in peace talks.

"We cannot allow violence and killings to continue while the parties are sitting here and negotiating," he said.

The Security Council has approved sending an extra 5,500 troops to South Sudan who are only slowly arriving.

UN peacekeeping chief Herve Ladsous said the reinforcements would allow UN forces, who are sheltering some 60,000 terrified civilians at bases across the country, "to go into a more pro-active footing around the bases and beyond because the situation in terms of violations of human rights remains terribly critical."

A senior US official said she feared for the country's future.

"Today, tragically, the world's youngest country and undoubtedly one of its most fragile democracies is in danger of shattering," US Assistant Secretary for Africa Linda Thomas-Greenfield told lawmakers in Washington.

"Each day that the conflict continues, the risk of all-out civil war grows as ethnic tensions rise."

National Security Adviser Susan Rice in a statement also called on both sides to "immediately sign" a proposed ceasefire deal but she singled out Machar, saying he "must commit to a cessation of hostilities without precondition."