WASHINGTON (AFP) - The United States (US) is pushing to get rival South Sudanese leaders to the negotiating table and end two weeks of bloodshed, but the situation remains complicated, a US official said on Monday.

US Secretary of State John Kerry has spoken almost every day over the past week with South Sudan's President Salva Kiir as well as sacked vice president Riek Machar in a bid to ease the tensions.

And special US envoy Donald Booth was on Monday in the capital, Juba, having spent Christmas in the region, trying "to finalise the details of a political dialogue, hopefully to arrange for negotiations to begin in the coming days", State Department deputy spokeswoman Marie Harf said.

"But it's a very complicated, tenuous situation."

The world's youngest nation, which only won independence in 2011, plunged into chaos on Dec 15 when Mr Kiir accused his former deputy of mounting a coup.

Bloodshed has swept across the nation, with fierce battles reported in strategic oil-producing areas and grim reports of massacres, rapes and killings.

So far the US has evacuated more than 400 US officials and private citizens, and some 700 people from 27 other countries on board seven chartered flights and six military aircraft, Ms Harf said.

Regional leaders at the Inter-Governmental Authority on Development (IGAD) have set Tuesday as a deadline for face-to-face talks between Mr Kiir and Mr Machar.

But Mr Machar has yet to agree to the dialogue.

Mr Kerry has warned both sides that the US would cut off its vital aid lifeline to the country if there is 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," Ms Harf said.

"And now is the time for all of these leaders who have helped with this process of creating a new nation, to take the right steps and make the right decisions."