South Sudan’s Vice President Has Been Arrested, Party Says

South Sudan’s Vice President Riek Machar has been arrested, according to his party, amid escalating tensions in the world’s youngest country that the United Nations warned could blow up a fragile peace deal and has pushed the country back to the brink of civil war.

Mr. Machar was detained late on Wednesday by the National Security Service, his acting press secretary, Puok Both Baluang, said. The Sudan People’s Liberation Movement In Opposition, Mr. Machar’s political party, said the country’s defense minister and the chief of national security “forcefully entered” Mr. Machar’s residence alongside an armed convoy, disarmed his bodyguards and “delivered an arrest warrant to him under unclear charges.”

It was not immediately clear where Mr. Machar was on Thursday morning, and spokespeople for the government and the opposition could not immediately be reached for comment.

“This act is a blatant violation of the Constitution,” Reath Muoch Tang, a senior official in Mr. Machar’s party, said in a statement posted on social media. He added that arresting Mr. Machar “without due process undermines the rule of law and threatens the stability of the nation.”

The arrest threatens the fragile peace agreement signed in 2018 between Mr. Machar and President Salva Kiir, which ended a five-year war that killed nearly 400,000 people.

The U.S. State Department said it was concerned about reports that Mr. Machar was “under house arrest” and in a post on social media called on President Kiir “to reverse this action & prevent further escalation of the situation.”