South Sudan's First Vice President Riek Machar has been charged with murder, treason and crimes against humanity for his alleged involvement in attacks by an ethnic militia against federal forces in March, the justice minister said on Thursday.

Hours after the minister's announcement, President Salva Kiir suspended Machar from his post, a decree read on state radio said, without giving more details.

The charges against Machar and his suspension escalate a feud between the country's two main political camps - the other led by Kiir - which faced off in a 2013-2018 civil war in which an estimated 400 000 people died.

Kiir also suspended Petroleum Minister Puot Kang Chol, who was charged alongside Machar, the decree said.

Machar has been under house arrest since March in connection with attacks by the White Army militia in the northeastern town of Nasir.

International powers have repeatedly called for Machar's release, warning his detention could tip the country back into civil war.

"Evidence further reveals that the White Army operated under the command and influence of certain leaders of the Sudan People's Liberation Movement/Army-in Opposition (SPLM/A-iO), including Dr. Riek Machar Teny," Justice Minister Joseph Geng told reporters.

"While the government of the Republic of South Sudan appreciates the engagement of international partners, public and media on the Nasir incident, this matter is now sub judice," he said.

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

Besides Machar, 20 other people were indicted in the case. Thirteen of those are at large, Geng said.

Edmund Yakani, executive director of Community Empowerment for Progress Organization, a South Sudan activist group, said in statement that he hoped the court trying Machar and the seven others would be fair and "a competent court of law, not a kangaroo court of law."