International guarantors of South Sudan's peace process said the transitional government's postponement of elections due in December was disappointing and showed its failure to implement a 2018 peace plan.

Last Friday, South Sudan President Salva Kiir's office announced an extension of the transitional period by two years and postponed elections for a second time following a delay in 2022.

South Sudan has formally been at peace since the 2018 agreement ended a five-year conflict responsible for hundreds of thousands of deaths, but violence between rival communities flares frequently.

Britain, the United States, Norway, the European Union, Canada, France, Germany and the Netherlands - guarantors of the peace process since South Sudan's independence from Sudan in 2011 - said the extension was a disappointment.

"This decision reflects the failure of the transitional government to implement the 2018 peace agreement despite the promises made with the release of the 'roadmap' in 2022," they said in a joint statement late on Wednesday.

They said elections were the best way to bring peace to the country and South Sudan's leaders must act urgently to create the necessary conditions for them to take place.

Michael Makuei, the information minister and government spokesperson, did not respond when called for comment.

The election would have picked leaders to succeed the transitional government, which includes Kiir and First Vice President Riek Machar, whose respective forces battled each other during the civil war.

The United Nations' special envoy for South Sudan, Nicholas Haysom, said in a statement it was evident that the country was not ready for elections.