The United Nations on Sunday said it was "extremely concerned" over claims that thousands of armed youths allied to ex-vice president Riek Machar were readying to attack in South Sudan, amid international efforts to broker a ceasefire.

Juba has claimed that Machar, the de facto leader of rebels fighting the government, has recruited up to 25,000 young fighters from his Nuer tribe in the eastern state of Jonglei and that they are "ready to attack any time".

According to Jonglei's acting governor Ogato Chan, the Nuer fighters were around 110 kilometres (70 miles) from the state capital of Bor, currently under the control of government forces.

"The information is that they want to come and attack Bor but I am sure they will not attempt to do it because the SPLA (Sudanese army) forces will repulse them back," Chan said. "Now the army are taking position."

Rebel spokesman Moses Ruai Lat said Machar was "not mobilising his tribe", the second biggest ethnic group in South Sudan, describing the men instead as regular soldiers who had rejected the government and were not specifically drafted by Machar.

A spokesman for the UN mission in South Sudan (UNMISS) told AFP it was "extremly concerned" about the reported advance of the armed youths towards Bor, saying it must be halted "to avoid further bloodshed".

The Nuer fighters are members of a tribal militia known as the "White Army" that became synonymous with years of violence and terror during the 1990s civil war.

Experts say the name comes in part from the ash the fighters use to protect their skin from insects.

In 2011 and 2012 the White Army turned on the Murle ethnic group, killing hundreds in a conflict over cattle theft.

"These youth have been reportedly moving across the state for some time now with the possible intention of attacking other communities," UNMISS spokesman Joseph Contreras said.

The United Nations could not yet independently confirm the size or location of the group nor its connection with forces loyal to Machar, he added.

"We do call upon all parties who can exercise influence over these armed groups to convince them to immediately halt their advance to avoid further bloodshed. These youth should return to their town, villages and cattle camp to avoid a further escalation of the crisis in South Sudan."

Claims that the White Army was being mobilised have cast a shadow over peace talks spearheaded by regional leaders to end two weeks of bloody violence believed to have killed thousands. The unrest has also raised fears that the country could slide into civil war.

Tuesday deadline for talks

Regional leaders at the Inter-Governmental Authority on Development (IGAD) have set Tuesday as a deadline for face-to-face talks between President Salva Kiir and his former deputy to end the fierce battles for control over several strategic oil-producing areas notably in the north of South Sudan.

While the government has said it was willing to observe a ceasefire, Machar -- who was sacked as vice-president in July -- has made demands including the release of his arrested political allies before committing to a truce.

South Sudanese government spokesman Michael Makuei told AFP Sunday: "I really doubt if we, the South Sudanese government, will be in a position to sit with Riek Machar... He has not even respected the call by IGAD and the African Union to agree to the cessation of hostilities."

Makuei said authorities were only ready to free eight of the 11 detainees linked to Machar, and only once the former vice-president had accepted a ceasefire agreement and talks had started.

Fuelled by an old rivalry between the two, the conflict has fanned ethnic differences between Kiir's Dinka group and Machar's Nuer clan in the country, which won independence from Sudan in 2011.

Fighting broke out on December 15 after Kiir accused Machar of mounting a coup, which his rival has denied. Rebels swiftly took over several key regional cities including Bentiu, in the northern oil-producing state of Unity, and Bor, which was later recaptured by the army.

In recent days, grim reports of massacres, rapes and killings nationwide have emerged.

Washington, which championed South Sudan's drive for independence, as well as the United Nations and Beijing have all pressed for dialogue, with UN chief Ban Ki-moon reiterating Saturday that all perpetrators of violence "will be held accountable".

More than 120,000 people are estimated to have been displaced by the clashes, according to the United Nations, which is to double its peacekeeping force to more than 12,000 to contain the bloodshed.

The UN refugee agency UNHCR is also investigating reports that hundreds of South Sudanese have fled to neighbouring Sudan.

South Sudan, the world's youngest nation, became independent after a civil war that killed more than two million people between 1983 and 2005.

AFP