South Sudan's opposition leader and suspended vice president Riek Machar appeared in court on Monday after being charged with crimes against humanity and treason, state media showed.

A fragile power-sharing deal between President Salva Kiir and his long-time rival, Riek Machar, has been unravelling for months, with risks of returning the young nation to a civil war that left 400,000 dead in the 2010s.

Machar was this month charged with murder, treason and crimes against humanity and suspended as first vice president in the unity government, having already spent months under house arrest.

He was accused of ordering an ethnic militia's attack on a military base in March that the government said killed more than 250 soldiers.

Machar's faction denies the charges and says they are part of Kiir's efforts to sideline the opposition and consolidate power.

"His Excellency Dr Riek Machar should not be tried by this incompetent court, which lacks jurisdiction," his lawyer told the court on Monday, as Machar appeared for the first hearing in the case alongside several other defendants. The case was adjourned to Tuesday, 23 September.

Failed transition

South Sudan gained independence from Sudan in 2011 but was quickly plunged into a devastating conflict between Kiir and Machar between 2013 and 2018.

Attempts by the international community to keep the peace process alive and ensure a democratic transition have largely failed.

Elections due to take place in December 2024 were again postponed to 2026, and the two sides have not merged their armed forces.

Machar's supporters say the charges against him show the power-sharing agreement is dead and have called for an armed mobilisation to bring about "regime change", AFP reports.