The detention of South Sudan's First Vice-President Riek Machar under house arrest has effectively collapsed the peace deal that ended the 2013-2018 civil war, his party said on Thursday.

The UN 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 five-year civil war, which was fought largely along ethnic lines, left hundreds of thousands of people dead in the country that gained independence from Sudan in 2011.

Oyet Nathaniel Pierino, deputy chair of Machar's Sudan People's Liberation Movement In Opposition (SPLM-IO) party, 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.

The SPLM-IO said South Sudan's defence minister and 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 that clashed with the military in Nasir, Sudan, 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 war.

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

"The deliberate targeting of opposition leaders and civilians represents a reckless disregard for international law and the country's future," Yasmin Sooka, the commission's chairperson, said in a statement.

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

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.

UN calls for commitment to peace

The army was heavily deployed near Machar's house on Thursday, a Reuters journalist said. On Wednesday, the UN reported fighting between forces loyal to President Salva 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."

The coalition government, in which Machar shares power with his rival Kiir, 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 Bol Mel was based on misleading information.

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