Martin Elia Lomuro, the minister of cabinet affairs, said the decision was taken "to address the challenges that impede the implementation of the peace agreement", following a 2018 deal to end a five-year civil war that left nearly 400,000 people dead.

"Thus a new roadmap has been agreed," the minister said, speaking in the presence of President Salva Kiir and Vice President Riek Machar, who formed a unity government more than two years ago after half a decade of fighting.

The world's newest nation was meant to conclude a transition period with elections in February 2023, but the government has so far failed to meet key provisions of the agreement, including drafting a constitution.

The so-called troika of the United States, Britain and Norway boycotted Thursday's announcement, pointing out that the government had not consulted all the parties involved in the 2018 deal before announcing the extension.

In a letter to Kiir, the troika expressed "profound concern that fully inclusive consultations must take place with civil society, faith-based groups, business, women's groups, youth representatives, eminent persons and international partners before the (peace deal) is amended".

"Whether a roadmap and an extension are seen as legitimate by the people of South Sudan and the international community will depend on an inclusive consultation process," the letter said.

"We cannot guarantee that we will be able to support a roadmap or extension in other circumstances."

In a speech addressing the nation, Kiir explained the decision, saying: "We don't want to rush you into an election that will take us back to war."

"We extend the transition period as a pragmatic and realistic choice for 24 months of healing and consolidating."

South Sudan's lumbering peace process has run into multiple delays, with violence breaking out between Kiir and Machar's forces as recently as this year.

The troika said the roadmap "must demonstrate how another extension would differ from previous ones, and include steps for clear progress in setting up the institutions and mechanisms necessary to hold elections".

- Alleged war crimes -

The United States last month pulled out of two peace process monitoring organisations in South Sudan due to the country's failure to meet reform milestones, citing a "lack of sustained progress".

After long delays, Kiir and Machar finally inked a deal on the creation of a unified armed forces command in April -- a key provision of the peace deal.

The United Nations, which maintains a peacekeeping mission in the country, has repeatedly criticised South Sudan's leadership for its role in stoking violence, cracking down on political freedoms and plundering public coffers.

The peacekeeping operation, with up to 17,000 soldiers and 2,100 police officers, is one of the UN's most expensive, with an annual budget topping $1 billion.

The UN has also accused the government of rights violations amounting to war crimes over deadly attacks in the southwest last year.

South Sudan, one of the poorest countries on the planet despite large oil reserves, has suffered from war, natural disasters, hunger, ethnic violence and political infighting since it gained independence in 2011.

The UN's World Food Programme warned in March that over 70 percent of South Sudan's 11 million people would face extreme hunger this year because of natural disasters and violence.