The world's newest nation has suffered from chronic instability since independence in 2011, including a brutal five-year civil war between forces loyal to President Salva Kiir and his deputy Riek Machar that cost almost 400,000 lives.

The 2018 agreement which ended the war has been bedevilled by bickering between rival parties, and key provisions of the deal are yet to be implemented.

Nicholas Haysom, the UN's special envoy to South Sudan, said the process risked becoming unviable if it failed to pick up the pace.

"The parties must inject fresh urgency into the process and demonstrate a sustained, collective political will to finalise critical areas of the agreement," he told a press briefing.

"Put simply, there cannot continue to be a "business as usual" approach."

Haysom also sounded an alarm on the country's severe flooding crisis, which has affected over 800,000 people following six months of torrential rain.

"I can only describe the situation as dire. Flood waters are not receding and hundreds of thousands of people remain displaced and in desperate need of assistance," he said.

South Sudan has struggled with war, famine and chronic political and economic crisis since celebrating its hard-fought independence from Sudan in July 2011.

As discontent has deepened, some citizens have called for a peaceful public uprising to topple the current regime, saying they have "had enough".

The shaky alliance between Kiir and Machar faced a fresh threat in August when deadly fighting erupted between rival factions of the vice-president's party, Sudan People's Liberation Movement/Army-In Opposition (SPLM/A-IO).

At least 32 people were killed in the clashes, with Machar saying the squabbles were aimed at derailing the formation of a unified armed forces command, a key component of the peace deal and a potential safeguard against future conflict.

Since the signing of the 2018 agreement, Machar has faced growing opposition within his own ranks, with top cadres complaining they had lost out to the ruling party.

The two parties have yet to agree on a power-sharing deal to unite their armies.