South Sudan's main opposition accused the government on Wednesday of failing to push through a peace deal and called for a six-month delay in the formation of a unity administration, casting a shadow over efforts to end years of fighting.

The spokesperson for former rebel leader Riek Machar said he did not believe he would be able to join a unity government on November 12 - a deadline agreed in September after months of talks, broken ceasefires and pressure from the United Nations, the United States and regional powers.

Speaking later at a public event, President Salva Kiir did not directly address the comments from Machar's camp. He said all parties to the agreement had committed to forming the unity government on November 12 and the international community expected that to happen.

"I want to welcome (the opposition) and forget all the bitterness," Kiir said.

There was no immediate comment from other countries that helped broker the accord. US officials said this month they would not accept more delays and might impose sanctions if deadlines are not met.

"It's not rocket science that the government in Juba lacks political will to implement the peace deal," Machar's spokesperson Puok Both Buluang said.

He called on the government to release funds it had agreed to spend on rolling out the accord. The extra six months would "give room" for resolving issues, he added.

Fighting is unlikely to resume on November 12, said Alan Boswell, a senior analyst with Brussels-based think-tank International Crisis Group.

Most of the international community is now urging both sides to agree on a new road map by the deadline in order to salvage the deal, he said.

South Sudan split from Sudan in 2011 after decades of war, then plunged into its own conflict at the end of 2013 after Kiir sacked Machar as vice president.

Subsequent fighting between troops loyal to both men shut down oil fields, forced a third of the country's population from their homes and killed more than 400 000.

The peace deal has stopped the fighting. But South Sudan's government says it cannot afford to fund disarmament and the integration of rebels into the army.

So far, it has allocated $10-million of the pledged $100-million, according to the international body monitoring the ceasefire.

Both sides have disagreed on details of the deal, including how many states South Sudan should have. Under the accord, they have agreed to hold elections after a three-year transition period.

A UN Security Council delegation visited Juba earlier this month to try to persuade the two sides to resolve their differences over the pact.

In a sign of how fragile South Sudan's peace is, the UN migration body IOM said on Wednesday it had suspended some of its screening services for Ebola after three of its aid workers were killed.

A military spokesperson said they were killed after fighting between government troops and fighters of the National Salvation Front, led by former General Thomas Cirillo Swaka, who is not a party to the peace deal.