ADDIS ABABA (Reuters) - South Sudanese President Salva Kiir and rebel commander Riek Machar signed another ceasefire agreement on Monday, edging them closer to a final deal to end a 15-month conflict that has ravaged the world's newest country, mediators said.

African diplomatic sources said the agreement, which has not been made public, sets out how the two leaders would share power once they formed an interim government. It is proposed Kiir would remain president while Machar would become vice president.

The warring sides also agreed to abide by a ceasefire deal signed in January 2013 but frequently violated.

The rebels, however, said many more details need to be ironed out before the deal can be labelled a "power-sharing" agreement.

After signing the latest agreement, Machar said the two sides would hold more discussions on the functions of the provisional government.

Few other details were revealed after frantic late-night talks. Regional diplomats had warned the warring sides that failure to come up with a new deal could see sanctions imposed on them.

The conflict in Africa's newest nation and one of its poorest erupted in December 2013 and has rumbled on since then despite several commitments by Kiir and Machar to halt the violence.

More than 10,000 people have been killed, about 1.5 million people have been driven from their homes and many in the oil-producing nation of about 11 million people are struggling to find enough food to eat.

Seyoum Mesfin, chief mediator of the East African IGAD bloc, said the two leaders had agreed to resume talks on Feb. 20.

"[Those talks] would be final and that would lead them into concluding a comprehensive agreement to end the crisis in South Sudan," Mesfin told reporters minutes before Kiir and Machar signed the latest peace deal.

Several previous peace deals and ceasefires that accompanied the agreements were swiftly broken.

The two sides need a transitional government in place by July, when Kiir's presidential term runs out.

Rights groups have said both factions have been responsible for ethnic killings and other abuses, driving the nation to the brink of famine. The fighting has largely pitted Kiir's Dinka ethnic group against Machar's Nuer group.