Negotiations in Addis Ababa, Ethiopia, are centered on a cease-fire and prisoner exchange, the BBC said.
Delegates began arriving Wednesday but decided not to begin talks until all members had arrived. Mediators making arrangements for direct negotiators held talks Friday.
For negotiations to have any success, South Sudan Foreign Minister Barnada Marial Benjamin said, rebel forces led by ousted Vice President Riek Machar would have to admit they had instigated a coup attempt. Machar said his forces would delay an attack on the capital Juba while trying to negotiate a settlement.
Jok Madur Jok of the Sudd Institute, an independent research institution in South Sudan, said the "most urgent" priority at the moment was a cease-fire, Voice of America reported. Those affected by the fighting are "extremely desperate," he said.
Jok said mediators are in an uncomfortable position because they want to signal the use of violence in the pursuit of political power "must be and should be discouraged at all cost." In doing so, they might condemn Machar's action, "but by doing so they risk pushing away into a kind of a civil war."
U.S. President Barack Obama, who has been briefed daily on the situation in South Sudan, has few good options, the New York Times said Friday.
The United States led the effort for South Sudan's independence from Sudan, and has threatened to cut off aid to any group that seizes power. Still, the State Department has pulled much of its staff from the U.S. Embassy in Juba and ringed the building with four dozen soldiers to prevent a repeat of the attack on the U.S. Consulate in Libya in 2012.
The administration pledged an additional $50 million in humanitarian aid Friday after receiving reports 180,000 people had been forced from their homes by the fighting.
Obama is unlikely to commit troops, said Tom McDonald, the U.S. ambassador to Zimbabwe under former President Bill Clinton. Still, he said, "we can't allow the carnage to go on ... . We have too much to lose, we've put too much into this."