South Sudan's President Salva Kiir has replaced its foreign minister with his deputy, Monday Simaya Kumba, state media reported, following a migration dispute with the United States.

No explanation was given for the sacking of Foreign Minister Ramadan Mohamed, which was announced on the state radio station late on Wednesday.

The move follows a row with Washington over Juba's refusal to admit a Congolese man deported from the United States, which led to the Trump administration threatening to revoke all US visas held by South Sudanese citizens.

South Sudan yielded to Washington's demands on Tuesday and allowed the man to enter the country.

Separately, a faction of South Sudan's main opposition party (SPLM-IO) said on Wednesday it had replaced its chairman, First Vice President Riek Machar, with an interim leader, Peacebuilding Minister Stephen Par Kuol, until Machar was released from house arrest.

The move, which was criticised by other members of the party, could allow Kiir to sack longstanding rival Machar and consolidate his power over the government by appointing Kuol, analysts said.

"President Kiir (would) want people who would agree with him ... so that now the government's legitimacy will be created," said Kuol Abraham Nyuon, professor of political science at the University of Juba.

Machar, who has served in a power-sharing administration with Kiir since a 2018 peace deal ended a civil war between fighters loyal to the two men, was accused of trying to stir up rebellion and detained at his home last month.

Machar's party denies government accusations that it backs the White Army, an ethnic militia which clashed with the army in the northeastern town of Nasir last month, triggering the latest political crisis.

African Union mediators arrived in Juba last week to try and rescue the peace deal, but did not appear to have made any immediate progress.

On Thursday, embassies based in Juba, including France, Germany, Netherlands, Norway, the United Kingdom, United States, the European Union, reiterated their call for the immediate release of all political detainees.

"It is urgent that South Sudan’s leaders meet their obligations and demonstrate that their priority is peace," they said in a joint statement.

The SPLM-IO said Machar's detention had effectively voided the agreement that ended the five-year civil war in which hundreds of thousands of people were killed. The party later said they were committed to upholding the deal.

The SPLM-IO's military wing remained loyal to Machar, and was "not part and parcel of the betrayers in Juba", its spokesperson Lam Paul Gabriel said in a statement on Wednesday.

Analysts say Kiir, 73, appears to be trying to shore up his position amid discontent within his own political camp and speculation about his succession plan.