The U.N. peacekeeping mission in South Sudan called for restraint, saying the country 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.

The civil war - fought between forces loyal to Machar and his rival, South Sudan's President Salva Kiir, often along ethnic lines - left hundreds of thousands of people dead. It ended in a deal that brought both men together in a fractious national unity government.

Machar's SPLM-IO party said South Sudan's defence minister and its chief of national security "forcefully entered" Machar's residence in the capital, Juba, on Wednesday evening to deliver an arrest warrant.

Machar was being held with his wife at his home, accused of supporting the White Army militia which clashed with the military in Nasir, Upper Nile State, this month, Reath Muoch Tang, a senior SPLM-IO official said in a statement.

Machar's party denies ongoing links with the White Army, which it fought alongside during the civil war.

South Sudan's army and government spokespeople did not immediately respond to requests for comment about Machar or his party's statement on the peace deal.

SPLM-IO deputy chair Oyet Nathaniel Pierino, said Machar's detention meant the agreement had been "abrogated".

It "effectively brings the agreement to a collapse, thus the prospect for peace and stability in South Sudan has now been put into serious jeopardy", he said.

William Ruto, president of neighbouring Kenya, said on X he had held a phone call with Kiir on Machar's arrest and detention and was sending a special envoy to Juba to try to defuse the situation.

Ruto said he had consulted with Yoweri Museveni, the president of Uganda, and Abiy Ahmed, the prime minister of Ethiopia - two other regional powers bordering South Sudan.

The U.N. Commission on Human Rights in South Sudan said the arrests marked an unravelling of the peace process.

The African Union and the Intergovernmental Authority on Development regional economic bloc also called for restraint.

US CALLS FOR COMMITMENT TO PEACE
The army was heavily deployed near Machar's house on Thursday, a Reuters journalist said. On Wednesday, the U.N. reported fighting between forces loyal to Kiir and Machar close to Juba.

The United States Bureau of African Affairs urged Kiir to release Machar and called on South Sudan's leaders to "demonstrate sincerity of stated commitments to peace".

South Sudan's coalition government has been slow to enact key provisions of the peace agreement, which include national elections and the unification of their two forces in one army.

Political analysts say Kiir has been attempting to shore up his position by rounding up some of Machar's most senior allies, inviting Uganda's army to secure the capital and naming adviser Benjamin Bol Mel as second vice president.

They say Kiir, 73, is preparing Bol Mel, a businessman on the U.S. sanctions list over his links to construction firms accused of money-laundering, to succeed him. South Sudan said at the time the decision to blacklist him was based on misleading information.

The United Nations had already warned that the violence in Nasir, around 450 km (280 miles) northeast of Juba, and a rise in hate speech could reignite the civil war along ethnic lines.

(Reporting by Nairobi Newsroom; Writing by Hereward Holland and George Obulutsa; Editing by Michael Perry, Alison Williams, Ammu Kannampilly and Andrew Heavens)