A fragile peace deal in South Sudan was collapsing on Thursday after the arrest of First Vice President Riek Machar, long-time rival to President Salva Kiir, threatened to throw the country back into war.

A convoy of 20 heavily armed vehicles entered Machar’s residence in the capital Juba late on Wednesday and arrested him, according to a statement issued by a member of his party–a dramatic escalation of a conflict that has been building for weeks in the world’s youngest country.

A power-sharing deal between Kiir and Machar has been gradually unravelling, risking a return of the civil war that killed around 400,000 people between 2013 and 2018.

The deputy chair of Machar’s party said his arrest had “abrogated” the agreement.

“The prospect for peace and stability in South Sudan has now been put into serious jeopardy,” said Oyet Nathaniel Pierino in a statement.

There was widespread international condemnation, including concern from the African Union and United Nations Mission in South Sudan (UNMISS), with the latter warning that the country was “on the brink of relapsing into widespread conflict”.

Juba was calm on Thursday, with businesses open and people on the streets, an AFP journalist said. But there was a heavy military presence around Machar’s home, which is located just metres (yards) from the president’s home, including a tank.

The population is deeply worried about what might come next, civil society leader Edmund Yakani said.

“The public are in a panic,” he told AFP.

“There is a high chance of full-scale war but it will be more deadly and more violent because of (the need) for revenge,” he added.

The US State Department on Thursday called on Kiir to “reverse this action and prevent further escalation” in a post on X.

Regional powers scrambled to mediate, with Kenyan President William Ruto posting that he had spoken to Kiir about Machar’s arrest.

After consultations with the leaders of Uganda and Ethiopia, Ruto said he was “sending a special envoy to South Sudan to engage, try (to) de-escalate, and brief us back.”

Uganda is already deeply involved in the conflict, having sent armed forces to support Kiir earlier this month. South Sudan–which declared independence from Sudan in 2011–has remained plagued by poverty and insecurity since the peace deal in 2018.

Analysts say an ageing Kiir, 73, has been seeking to ensure his succession and sideline Machar politically for months through cabinet reshuffles.

More than 20 of Machar’s political and military allies in the unity government and army have also been arrested since February, many held incommunicado.

Machar’s party says three of its military bases around Juba have been attacked by government forces since Monday.

The training centres were established to prepare opposition forces for integration into the unified army — a key provision of the 2018 peace agreement aimed at uniting government and opposition troops.

None of the incidents have been confirmed by the Kiir-aligned army, the South Sudan People’s Defence Forces (SSPDF), although it accused Machar’s forces of aggressive manoeuvres from one of the bases on Monday. 

Prior to Machar’s arrest, Kiir said he reaffirmed his “unwavering commitment to restoring peace” following a meeting with church leaders.