Machar made an impassioned plea to a visiting United Nations Security Council delegation that met with him and President Salva Kiir to urge speedier progress in pulling the country out of a five-year civil war that killed almost 400,000 people.
"Suppose we form a government on the 12th, you know what's going to happen? The ceasefire we've been enjoying for over a year will erupt," said the visiting Machar, whose opposition has said he won't return to South Sudan for good until security arrangements are in place.
The previous attempt at Kiir and Machar sharing power ended in renewed fighting and Machar fleeing the country on foot in 2016. The issues being discussed today are the same ones that led to that earlier failure, the opposition leader said.
But the U.S. ambassador to the U.N., Kelly Craft, said the council was "disappointed" by Machar's warnings. The U.S. has said it would reevaluate its relationship with South Sudan if next month's deadline isn't met.
The Security Council still wants the Nov. 12 deadline met, South Sudan government spokesman Michael Makuei said: "No change of schedule nor change of program."
The opposition, however, now wants a three-month delay, Makuei said.
One key outstanding issue is security. Machar and Kiir were told Sunday that it would take at least three months to train at least 41,500 fighters and troops into a unified national army along with a 3,000-member VIP protection force.
South Sudan's government has said the international community should help fund that process.
The Security Council "is of the view that nothing is impossible, nothing is unsurmountable," said South Africa's ambassador to the U.N., Jerry Matthews Matjila. The remaining issues can be discussed by an inclusive government, he said.
One South Sudan expert said the international community is making a mistake.
"The U.N. Security Council took the wrong approach today," Alan Boswell, senior analyst with the International Crisis Group, told The Associated Press.
"Rather than fixate on Nov. 12, everyone should be focused on pressuring the parties to resolve the issues necessary to form a viable government at less risk of collapse," he said. "There are much worse scenarios than another delay."
Another expert suggested a different approach.
South Sudan's government "has consistently acted in bad faith," said Klem Ryan, former coordinator of the Security Council's panel of experts monitoring sanctions against South Sudan. "They need to be treated as illegitimate through increased international isolation by the diplomatic community until such time as they show a genuine desire to meet the needs of the people of South Sudan."
The world's youngest nation erupted in civil war just two years after winning independence from Sudan as Kiir and his deputy, Machar, clashed and their supporters took up arms. Millions have since been displaced and the oil-rich country's economy has been shattered.