The ongoing fighting between armed groups aligned with the government and others belonging to opposition forces in South Sudan in 2021 subjected civilians to "unimaginable violence", which might be equal to 'war crimes', according to a report by Amnesty International.

Fighters were documented indiscriminately murdering and mutilating civilians and wholly demolishing villages during an outbreak of fighting between June and October in the Western Equatoria region.

Tambura County witnessed ethnically-driven clashes after politicians brought up local grievances and encouraged young people to take up arms, Amnesty said in a new report.

"That the attacks not only involved local groups, but also fighters affiliated to government and opposition forces, indicates this is much more than inter-communal violence," the rights watchdog said.

"The testimonies we have gathered speak of unimaginable violence, including civilians killed as they fled and bodies set on fire and mutilated," said Amnesty's Regional Director, Deprose Muchena.

Witnesses told Amnesty that fighters "summarily killed civilians by shooting them or slitting their throats," sometimes in front of their children and other family members.

"Deliberately targeting civilians and the murder of captives are war crimes," Amnesty said.

After South Sudan's independence in 2011, the country plunged into a brutal civil war, resulting in the death of 400,000 people and forcing millions to flee.

Despite the signed ceasefire, key provisions of the peace deal have not been honored, and fighters loyal to each man remain bitterly divided.