The main opposition in South Sudan has hit out at the government saying it has failed to push through a peace deal and called for a six-month delay in the formation of a unity administration.

The spokesman for former rebel leader Riek Machar said on Wednesday he did not believe he would be able to join a unity government on Nov. 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 Nov. 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 who helped broker the accord. U.S. 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 the political will to implement the peace deal," Machar's spokesman 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.

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.