Kenya is sending former Prime Minister Raila Odinga to South Sudan as a special envoy to help defuse a growing rift between President Salva Kiir and his longtime rival First Vice President Riek Machar which threatens to drag the country back to war.

Machar has been under house arrest in the capital Juba since Wednesday night, his party says, effectively voiding a 2018 peace deal that ended a five-year civil war and brought the two men into a fragile power-sharing government.

Their administration has been slow to adopt key provisions of the peace pact, such as national elections and the unification of their two forces into one army.

Machar's detention took "the country one step closer to the edge of collapse into civil war," a spokesperson for United Nations Secretary-General Antonio Guterres said on Thursday.

Machar's party denies government accusations that it backs the White Army, an ethnic militia largely comprised of Nuer youths, which clashed with the army in the northeastern town of Nasir this month, triggering the latest political crisis.

In response to the fighting, Kiir's forces rounded up several of Machar's most senior allies, including the petroleum minister and the deputy head of the army.

Forces loyal to the two men have clashed in recent days, outside Juba, among other places.

Kenyan President William Ruto, who chairs the East African Community (EAC) bloc, said he had spoken with Kiir about Machar's detention, and was sending a special envoy to help de-escalate the situation and report back.

Odinga's spokesperson Dennis Onyango confirmed that the former prime minister would travel to Juba on Friday.

Ruto said he had also consulted Uganda's President Yoweri Museveni, who sent troops this month to South Sudan at the government's request to help secure the capital, and Prime Minister Abiy Ahmed of Ethiopia, which has hosted South Sudan peace talks in the past.

Rival factions loyal to Kiir and Machar fought a civil war from 2013 to 2018 that killed hundreds of thousands.