The political party of South Sudan's First Vice President Riek Machar has warned that his arrest on Wednesday invalidates the country's 2018 peace deal.
First Vice President Riek Machar, South Sudan's opposition leader, is reportedly under house arrest.
Machar's Sudan People's Liberation Movement-in-Opposition (SPLM-IO) party said that it was trying to locate him, after that the defence minister and chief of national security "forcefully entered" Machar's residence and delivered an arrest warrant on Wednesday evening.
Machar was then held at his house overnight with his wife and two bodyguards.
He is accused of being implicated in fighting between the military and the White Army, an ethnic Nuer militant organisation, in Nasir, Upper Nile State this month, senior SPLM-IO official Reath Muoch Tang said in a statement.
The party added on Thursday that the arrest of Machar, long-time rival to President Salva Kiir, had invalidated the 2018 peace deal and risked plunging the country back into war.
Under the deal, which ended the 2013 to 2018 civil war between forces loyal to Machar on one side and Kiir on the other, South Sudan has five vice-presidents. Machar is currently serving as first vice-president.
The United Nations has warned that the recent clashes in Nasir between the army and the White Army, a militia with historical ties to Machar, along with a rise in hate speech, could reignite the civil war.
Machar's SPLM-IO party denies ongoing links with the White Army.
South Sudan's army and government spokespeople did not immediately respond to requests for comment.
Rumours of Machar's imminent arrest have been rife since early March, according to RFI's correspondent in the country, when the crisis in the Upper Nile region in the northeast of the country led to the arrest of several officials from his party in Juba.
The UN peacekeeping mission in South Sudan (UNMISS) on Wednesday called for "an immediate cessation of hostilities and urgent dialogue between the country’s leaders to de-escalate tensions and restore calm".
"To prevent a relapse into civil war, the parties must recommit to the Revitalised Peace Agreement by ceasing all hostilities and strictly adhering to the ceasefire, resolving grievances through dialogue, and reconvening as a truly unified government,” said the head of UNMISS, Nicholas Haysom, in a statement.
He warned that "a return to widespread conflict... will devastate not only South Sudan, but the entire region".
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.
The United States on Thursday called on Kiir to release his rival. "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 (formerly Twitter).
"We urge President Kiir to reverse this action and prevent further escalation of the situation. It is time for South Sudan's leaders to demonstrate sincerity of stated commitments to peace," the post continued.
The African Union also released a statement on Machar's arrest on Thursday, saying: "The chairperson of the African Union Commission, H.E. Mahmoud Ali Youssouf, expresses deep concern over reports regarding the detention of the First Vice President of the Republic of South Sudan, Dr Riek Machar."
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.
(with newswires)