* Rebel leader does not commit to face-to-face talks
* Kerry on first trip as secretary of state to South Sudan
* World fears descent into genocide (Recasts with Machar call, Kiir comments, more details)
By Phil Stewart
JUBA, May 2 South Sudan's President Salva Kiir said on Friday he was ready for face-to-face talks with rebel leader Riek Machar to try and end months of fighting in the world's newest nation, but his rival held off from promising to take part.
Kiir spoke hours after U.S. Secretary of State John Kerry met him in South Sudan's capital Juba to urge him to help end the conflict - part of a diplomatic push by Western and African powers who fear it could tip into full-blown ethnic slaughter and destabilise an already fragile region.
"In the interest of peace in our country, I am willing and ready for face-to-face talks with Machar," Kiir was quoted as saying in a statement released by the government of Kenya, where he flew to brief his regional counterparts after meeting Kerry.
Thousands have been killed and more than 1 million people have fled their homes since fighting erupted in December between troops backing Kiir and soldiers loyal to Machar, his sacked deputy.
The violence, which broke out after a long political rivalry between the two men, quickly spread to areas including the oil-producing north, often along ethnic lines between Kiir's Dinka people and Machar's Nuer.
A senior U.S. State Department official said a phone conversation Kerry held on Friday with Machar was inconclusive.
Machar "expressed openness to participating" in talks but did not commit during the call with Kerry, the official said, speaking on condition of anonymity.
Kerry, on his first visit as secretary of state to South Sudan, warned on Thursday that the increasingly ethnic violence could descend into genocide and said he expected the rapid deployment of more peacekeepers.
On Friday, he cited the risks of famine in South Sudan, decried reported recruitment of child soldiers and cited "appalling accounts of sexual violence."
"Before the promise of South Sudan's future is soaked in more blood, President Kiir and the opposition must work immediately for a cessation of hostilities and move towards an understanding about future governance of the country," Kerry told reporters in the South Sudanese capital Juba.
Delegations from both sides have been meeting in neighbouring Ethiopia, but their talks have failed to advance since the Jan. 23 signing of a ceasefire that never took hold.
Kerry said a meeting between Kiir and Machar, which would be their first face-to-face encounter since the conflict began, would be "critical" to finally implementing the ceasefire. He said such talks might take place as early as next week, though that was before he had spoken with Machar.
Kerry also said Kiir had committed in their meeting "to take forceful steps" to end the violence as well as to engage in talks on a transitional government.
The top U.S. diplomat did not spell out what form such a transitional government might take or provide details of what it might mean for Kiir's continued leadership of the country.
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, failure to build political institutions have eroded much of the political goodwill in Washington towards the new country's leadership.
Oil output, South Sudan's economic lifeline, has been cut by a third to about 160,000 barrels per day since fighting began.
Kerry said he hoped an initial new deployment of about 2,500 U.N.-mandated, African troops could be sent in the coming weeks, bolstering the roughly 10,000 U.N. peacekeepers already there.
Western diplomats have said U.N. forces have needed a tougher mandate than the one under which the existing U.N. Mission in South Sudan (UNMISS) operates so they can act more assertively to halt violence and protect civilians.
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 U.N. base where Nuer were sheltering.
The United Nations' special adviser on prevention of genocide, Adama Dieng, reiterated concerns on Friday that the conflict in South Sudan could spiral into genocide.
(Additional reporting by James Macharia in Nairobi and Carl Odera in Juba; Editing by Andrew Heavens and Gareth Jones)