The United States blamed South Sudan's government on Sunday for the latest hold-up in efforts to end its civil war, but warned both sides the world will not tolerate backsliding.

Rebel leader Riek Machar had been due to fly to Juba from Ethiopia on Saturday to join a transitional government, but President Salva Kiir's government blocked his flight.

He is now due to return on Monday, but US special envoy to South Sudan Donald Booth said repeated delays had called into question both parties' commitment to the agreement.

"This marks the third time this week that plans for Machar to return ... have been frustrated by one side or the other," the senior diplomat told AFP in Washington.

"Machar frustrated the first two attempts by a last minute insistence on bringing additional security forces and additional heavy weapons -- RPGs -- with him," he said.

"Then yesterday it was the government that closed the airport," he added.

"The implementation of the peace agreement requires the formation of the transitional government -- and that requires the return of Riek Machar to Juba."

Booth warned that the United Nations Security Council is due to meet on Tuesday to discuss the crisis and that both sides could face additional international sanctions.

"They are definitely on notice," he said. "The entire world is expecting them to live up to their word and to implement the agreement that they signed last August."

Civil war erupted in South Sudan, Africa's youngest nation, in December 2013 when Kiir accused his former deputy Machar of plotting a coup.

Tens of thousands of people have been killed and more than two million been driven from their homes in a conflict that has reignited ethnic divisions.

Machar fled Juba when war broke out but has since agreed to forge a transitional unity government with arch-rival Kiir, returning to his former post of vice-president.

Booth said Kiir's government had at the last moment raised questions about screening for the weapons that Machar's 195 men would be allowed to bring with them.

But he said two former Ethiopian officers had been mandated by ceasefire monitors to check the group's AK-47 assault rifles, machine guns and rocket-propelled grenades.

"There really was no excuse for this," he told AFP.

"So it really does call into question the activity over the past week and the real willingness of both parties to proceed with this."

Asked whether Washington now believes Juba's assurances that the flight can go ahead next week, Booth was cautious.

"Well, they've come up with reasons one side or the other for this not to happen," he said. "They are going to have to work this out between the two of them."

South Sudan became independent from Sudan in 2011 after many years of fighting with northern loyalist forces, and the United States has long been a key supporter.

But the young country's rapid descent into in-fighting has frustrated its international backers and caused a terrible humanitarian disaster and refugee crisis.

A 1,370-strong armed rebel force has arrived in Juba as part of the peace deal and government forces say they have pulled all but 3,420 of their troops from the city.

All other combattants will have to remain at least 25 kilometers (15 miles) outside the capital.