The United States on Thursday called on South Sudan President Salva Kiir to release his rival First Vice President Riek Machar who was reportedly under house arrest, saying it was time the country's leaders demonstrated their commitment to peace.

Machar's SPLM-IO party said on Wednesday that the defence minister and chief of national security "forcefully entered" Machar's residence and delivered an arrest warrant.

Machar was being held at his house with his wife and two body guards, accused of being implicated in fighting between the military and White Army in Nasir, Upper Nile State this month, Reath Muoch Tang, a senior SPLM-IO official said in a statement seen by Reuters on Thursday.

"We are concerned by reports South Sudan's First Vice President Machar is under house arrest," Washington's Bureau of African Affairs wrote on X.

"We urge President Kiir to reverse this action & prevent further escalation of the situation."

Under a peace deal which ended a 2013-2018 civil war between forces loyal to Machar on one side and Kiir on the other, South Sudan has five vice-presidents. Kiir's longtime rival and opposition leader Machar is currently serving as first vice-president.

The United Nations has warned that recent clashes in Nasir between the army and the White Army, a militia with historical ties to Machar, and a rise in hate-speech could reignite along ethnic lines the civil war which ended in 2018.

Machar's SPLM-IO party denies ongoing links with the White Army.

"It is time for South Sudan's leaders to demonstrate sincerity of stated commitments to peace," Washington's Bureau of African Affairs wrote on X.

South Sudan's army and government spokespeople did not immediately respond to requests for comment.

CALL FOR RESTRAINT

Political analysts say that the peace deal, under which Kiir and Machar have been serving in a fragile coalition government, is on the brink of collapse.

The UN peacekeeping mission in South Sudan (UNMISS) called for restraint, saying that the country's leaders stood on the brink of relapsing into widespread conflict.

"This will not only devastate South Sudan but also affect the entire region," UNMISS said in a statement.

Earlier this month Kiir's government detained several officials from Machar's party, including the petroleum minister and the deputy head of the army, in response to the clashes with the White Army in Upper Nile State.

On Wednesday the UN reported fighting between forces loyal to Kiir and Machar close to the capital Juba.

The 2013-2018 civil war, which was fought largely along ethnic lines, resulted in hundreds of thousands of deaths in the world's youngest nation.