South Sudan's President Salva Kiir has agreed to fly to Ethiopia for talks to try and end four months of fighting in the world's newest nation, US Secretary of State John Kerry has said.

Mr Kerry, who met the president in South Sudan's capital Juba, said he hoped Mr Kiir would be able to hold a face-to-face meeting with rebel leader Riek Machar as early as next week to settle a conflict which has already killed thousands.

The US diplomat did not spell out what form a potential transitional government might take, or go into any details of what it might mean for Mr Kiir's continued leadership of the oil-producing country.

Mr Kerry's meeting came a day after he warned that the increasingly ethnic violence could descend into genocide and said he expected the rapid deployment of more peacekeepers.

He also said Mr Kiir had committed "to take forceful steps in order to begin to move to end the violence and implement the cessation of hostilities agreement and to begin to engage on a discussion with respect to a transition government."

Delegations from both sides have been meeting in neighbour Ethiopia but their discussions have failed to advance since the signing of a ceasefire that never took hold on 23 January.

"I just spoke a couple of minutes ago to the Prime Minister of Ethiopia to convey to him President's Kiir's willingness to (travel to) Addis Ababa, sometime early next week, hopefully in order to engage in a discussion...with Mr Machar," Mr Kerry told reporters.

Mr Kerry said that meeting, which would be Mr Kiir and Mr Machar's first face-to-face encounter since the start of the conflict, would be "critical" and he hopes to speak to Mr Machar later today to persuade him to take part.

More than 1 million people have fled their homes since fighting erupted in December between troops backing Mr Kiir and soldiers loyal to Mr Machar, his former deputy.

The violence quickly spread to areas including the oil-producing north, often along ethnic lines between Mr Kiir's Dinka people and Mr Machar's Nuer.

Oil output, the nation's economic lifeline, has been cut by a third to about 160,000 barrels per day since fighting began.

International fears of a descent into genocide grew after the United Nations said rebels massacred hundreds of civilians in the northern oil town of Bentiu last month.

Days later, residents of Bor, a predominantly Dinka town ,attacked a UN base where Nuer were sheltering.

Mr Kerry said he hoped an initial deployment of about 2,500 UN-mandated troops could be deployed within the next few weeks, to bolster some 7,000 UN peacekeepers already there.

"We need to secure an additional United Nations' Security Council mandate. I believe that can be done quickly," he said on his first visit to South Sudan as Secretary of State.

Western diplomats have said UN forces need a tougher mandate than the one under which the existing UN Mission in South Sudan (UNMISS) operates so they can act more assertively to halt violence and protect civilians.

South Sudan won its independence from Sudan in 2011 under a peace deal to end decades of conflict. Washington and South Sudan's neighbours played a central role in that process and have been scrambling to stem the latest violence.

Concerns about continued violence, rampant corruption and failure to build political institutions have eroded much of the political goodwill in Washington towards the new country's leadership.