Kiir and rebel chief Riek Machar, whose fallout in 2013 sparked a conflict that has left hundreds of thousands dead, were granted an extension of 100 days to form a power-sharing government on Thursday, in the latest modification to the truce they signed in September 2018.

"The 100 days extension is endurable, if that will stop the return of South Sudan to war," Kiir said in an address to the country, calling on all parties to be "patient".

A key aspect of the accord was that fighters from all sides would be gathered into military camps and trained as an 83,000-strong unified army -- but that plan has been dogged by delays and lack of funding.

"I want to assure you that we will raise the remaining part of the $100 million the government pledged," Kiir said.

"I call on the army and the other organized forces to maintain peace."

Kiir and Machar were granted the extension after a rare face-to-face meeting held with regional heavyweights in Uganda.

They had agreed to join forces in a coalition government by November 12. But with the date looming and key issues far from resolved, regional leaders brokered high-level mediations in Entebbe to chart a way forward.

The delay triggered a sharp warning from the United States, a major backer of the impoverished nation.

"This inability to meet their own deadline calls into question their suitability to continue to lead the nation's peace process," Tibor Nagy, the top US diplomat of Africa, said of Kiir and Machar.

"The US is considering all possible options to put pressure on those individuals who would impede peace and promote conflict," he wrote on Twitter.

The United States has previously threatened targeted sanctions without a prompt government formation, although an official earlier ruled out ending Washington's roughly $1 billion in humanitarian assistance.

The peace deal has largely stopped the fighting that erupted just two years after South Sudan achieved independence, violence that left nearly 400,000 dead and displaced close to four million people.