South Sudan’s President Salva Kiir and rebel chief Riek Machar were given another 100 days to form a power-sharing government on Thursday after failing to resolve differences.
But the delay prompted a sharp US warning that the fledgling nation needed new leaders.
The two rivals, whose fallout in 2013 sparked a conflict that has left hundreds of thousands dead, were granted the extension after a rare meeting held with regional heavyweights in Uganda.
It is the second time the deadline has been pushed back since the two signed a truce last September that brought a pause to fighting.
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.
"It was really impossible to have them reach agreement in five days," Ugandan Foreign Minister Sam Kutesa told AFP after the closed-door discussions at State House in Entebbe.
"We've given them three months and we will continue our engagement.
The meeting "agreed to extend the pre-transitional period and to review progress after 50 days from that date", Mr Kutesa.
The US, a major backer of the impoverished African nation, voiced frustration at the delay and said it would "review our relationship" with South Sudan's government.
"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 in Africa, said on Twitter.
"The US is considering all possible options to put pressure on those individuals who would impede peace and promote conflict."
Deeply disappointed President Kiir and Dr. Machar are delaying formation of a unity government in #SouthSudan. This inability to meet their own deadline calls into question their suitability to continue to lead the nation’s peace process. #respecttheceasefire 1/3
— Bureau of African Affairs (@AsstSecStateAF) November 7, 2019
The US had threatened sanctions without a prompt government formation, although an official earlier ruled out ending Washington's $1 billion (Dh3.67bn) in humanitarian assistance.
Mr Nagy voiced appreciation for the African mediators who included Ugandan President Yoweri Museveni, Gen Abdel Fattah Al Burhan, who leads neighbouring Sudan's sovereign council; and Kalonzo Musyoka, a special envoy from Kenya.
Despite US frustration at the slow progress, some observers warned that pushing the foes to form a unity government before disagreements over security and state boundaries were resolved threatened to plunge the country back into war.
The peace deal has largely stopped the fighting that erupted two years after South Sudan achieved independence. The violence left nearly 400,000 dead and displaced almost four million people.
"Another extension is far preferable than a return to conflict," said Alan Boswell, senior analyst at the International Crisis Group think tank.
"Regional mediators must step up at the highest levels to finally resolve the sticking points blocking the peace process from moving forward."
Mr Machar, who lives in exile in Khartoum and cannot travel freely in the region, asked for more time so that the impasse over security and territory arrangements could be overcome.
The rebel leader warned that if these were not addressed, the country would see a repeat of fighting in 2016, when an earlier peace deal collapsed, worsening the conflict.
A former deputy to Mr Kiir, Mr Machar fled South Sudan on foot under a hail of gunfire, and has only returned home on rare occasions, fearing for his safety.
Mr Kiir said he was ready to form a new government and had threatened to do it alone.
Creation of the coalition government, a key part of the September 2018 peace deal between the rivals, had already been delayed once in May when regional leaders brokered a six-month extension.
The UN Security Council, on the eve of the Entebbe meeting, declared that fully implementing "all provisions of the peace agreement remains the only path that will set the country towards the goal of peace, stability and development".
A cornerstone of the accord was that fighters from all sides would be gathered into military camps and trained as a unified army, but the process was dogged by delays and lack of funding.
Little progress has been made on negotiations around state boundaries, another major sticking point.
The EU on Thursday, before the extension was granted, urged the warring parties to demonstrate "genuine will to build peace" and set realistic deadlines for resolving outstanding issues.
