South Sudan's opposition has accused the government of trying to enforce "authoritarian control and one-tribe rule" after First Vice President Riek Machar was charged with orchestrating militia attacks and suspended from his role.

Machar's SPLM-IO party rejected the charges against him and 20 others, which included murder, treason and crimes against humanity for their alleged involvement in raids by the White Army militia in the northeast in March.

Machar's detention under house arrest since March has ignited international fears of a renewal of a devastating 2013-2018 civil war between his ethnic Nuer forces and Dinka fighters loyal to his longtime rival, President Salva Kiir.

Kiir and Machar served in a unity government as part of a peace deal that ended the war, but their partnership remained strained and sporadic violence has continued between the two sides.

"The charges are fabricated to abrogate the (peace agreement), sideline Dr. Machar and the SPLM-IO, and entrench total government control," Machar's SPLM-IO party said in a statement late on Thursday shortly after the justice ministry announced the charges.

Political analysts say Kiir has long been seeking to replace Machar with his close ally, Second Vice President Benjamin Bol Mel, who was sanctioned by the U.S. over suspicions that he received preferential treatment in securing government contracts.

South Sudanese officials have asked the U.S. to lift those sanctions during recent bilateral discussions, Joseph Szlavik, a lobbyist working for Juba in Washington, told Reuters last month.

Those conversations have also touched on sending more U.S. deportees to South Sudan following the arrival in July of eight men, including seven from third countries, Szlavik said.