The deal is to end a devastating war that erupted in December 2013 after Kiir fell out with Machar, who was his deputy.

The UN Security Council expressed “serious concern” on Tuesday over delays in the return of South Sudan’s rebel leader Riek Machar to the capital Juba as part of a peace deal.

The 15-member council met behind closed doors at the request of the United States to hear a report on the latest hurdle in the way of the agreement to end the two-year civil war.

Council members “urged all parties to quickly form the transitional government and fully implement the peace agreement,” said Chinese Deputy Ambassador Wu Haitao, whose country holds the council presidency.

UN peacekeeping chief Herve Ladsous told the council that there had been disagreements over security arrangements allowing rebel forces to be stationed in Juba in advance of Machar’s arrival, which was supposed to have been on Monday.

But he said he was hopeful that Machar would return on Wednesday, diplomats said.

The council called on all sides to “remain calm” and said it was “ready to address any obstruction of implementation of the agreement,” although it did not specify which measures were envisaged.

Under the peace deal, Machar was to join President Salva Kiir in a new 30-month transitional government leading to elections.

The deal is to end a devastating war that erupted in December 2013 after Kiir fell out with Machar, who was his deputy.

“The United States is extremely disappointed that Riek Machar has not fulfilled his commitments under the peace agreement and returned to Juba as he stated publicly he would,” said US Deputy Ambassador David Pressman.

“We expect Riek Machar and all parties to live up to their commitments under the peace agreement and do their part to establish the transitional government.”

Tens of thousands of people have died in the fighting and more than two million have been driven from their homes.

Machar was supposed to return to Juba for the first time since the war began from his tribal stronghold of Pagak in the east of the country to formally take up his post as vice president.

Speaking to reporters at Juba airport, rebel spokesmen William Ezekiel said on Tuesday that unspecified “issues relating to logistics” were to blame for the latest delay. He was unable to say when Machar might arrive.

South Sudan’s information minister Michael Makuei said the government had blocked Machar’s flight because he wanted to bring “machine guns and laser-guided missiles” as well as additional troops in violation of the peace agreement.

Various rebel officials have given differing explanations for the delays, with some citing difficulties in getting Machar’s bodyguards’ weapons across the border while others blamed bad weather.

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