JUBA (AFP) - Thousands have fled the flashpoint town of Bor in South Sudan fearing an impending attack by rebel forces, as hopes faded that a Tuesday deadline for peace talks would be met in the strife-torn nation.

A feared ethnic militia known as the "White Army" was reportedly marching on Bor, raising the spectre of further bloodshed as the international community scrambled to broker a truce after two weeks of fighting in the world's youngest nation.

Washington warned of a "very complicated, tenuous situation" as it toiled to get rivals President Salva Kiir and sacked vice president Riek Machar to the negotiating table.

A deadline for face-to-face talks between the two, set by regional leaders, seemed unlikely to be met Tuesday.

State Department deputy spokeswoman Marie Harf said late Monday that a special US envoy was in Juba trying "to finalise the details of a political dialogue, hopefully to arrange for negotiations to begin in the coming days." Ugandan President Yoweri Museveni has warned that Machar must comply with the ceasefire deal by Tuesday or face action by regional nations.

If Machar does not respond "we shall have to go for him" Museveni told reporters, without clarifying if his threat involved military action.

The country, which only achieved independence in July 2011, plunged into chaos on December 15 when Kiir accused his former deputy of mounting a coup.

The conflict has fanned ethnic differences between Kiir's Dinka group and Machar's Nuer clan.

Thousands are feared dead, with fierce battles reported in strategic oil-producing areas and grim reports of massacres, rapes and killings.

While the government has said it was willing to observe a ceasefire, Machar has set demands including the release of his arrested political allies before committing to a truce.

Heavy fighting has continued in oil-producing states such as Unity and Upper Nile, and the focus has now fallen on the strategic town of Bor, the capital of Jonglei state.

"The forces of Riek Machar are now advancing on Bor, but we are confident we will hold them off and protect the town," army spokesman Philip Aguer told AFP. "The people in Bor are fearing an attack at any time."

Reconnaissance flights by the UN Mission in South Sudan (UNMISS) on Sunday identified armed groups some 50 kilometres northeast of Bor but could not confirm the force's size.

The gunmen, a loose ethnic militia force loyal to Machar and dubbed the "White Army", are heavily armed - some carrying automatic rifles or spears, others armed with rocket propelled grenades. They are known for smearing white ash onto their bodies as war-paint and to ward off insects.

The UN said thousands of people were fleeing the city, which has a bitter history of conflict between the Nuer and the Dinka. In 1991 an estimated 2,000 people were killed in a massacre in Bor blamed on the Nuer.

"Thousands of civilians have been seen heading south on the road to Juba during the course of today, motivated one assumes primarily by fears of an assault on Bor by these groups," UN spokesman Martin Nesirky told reporters.

The Doctors Without Borders group warned of "catastrophic" conditions as more than 70,000 people, mostly women and children, had arrived from Bor in Awerial in neighbouring Lakes state.

"There is no clean water, no food, and no place to shelter or sleep," said Aurelie Dupont, the group's emergency coordinator in Awerial.

"They are relying solely on the help of the local population." Nesirky said the United Nations was "extremely concerned about mounting evidence of gross violations of international human rights laws" across South Sudan.

UN leader Ban Ki-moon spoke to Kiir on Monday and called on him to carry out a promise to release political prisoners.

US Secretary of State John Kerry has spoken almost every day over the past week with the two rivals in a bid to ease tensions. He has warned both sides that Washington would cut off its vital aid lifeline to the country if there was any bid to seize power by force.

"South Sudanese leaders have a choice here. They can choose to end the violence. They can chose to work to resolve these tensions peacefully," his spokeswoman Harf said.

The UN says some 75,000 have sought refuge in badly overstretched peacekeeper bases and more than 180,000 are displaced across the country.

Thousands of foreign workers have fled the country, including oil company employees in an industry which accounts for more than 95 percent of South Sudan's fledgling economy.

Even war-torn Somalia sent in a special plane Monday to evacuate its citizens.