A report released late Tuesday by the African Union said mass graves and evidence of horrific crimes, including forced cannibalism, have been discovered in South Sudan. The report said hundreds of members of the ethnic Nuer were captured, tortured and killed by alleged government forces or their allies, according to the Associated Press.

The African Union investigators behind the report, led by former Nigerian President Olusegun Obasanjo, accused South Sudanese President Salva Kiir’s party of recruiting an irregular tribal force before violence broke out between ethnic Dinka and Nuer soldiers of the presidential guard in the capital Juba, on Dec. 15, 2013. Government troops then committed organized, mass killings of Nuer civilians and soldiers, the report said.

The conflict sparked from building political tension between Kiir, a Dinka, and his deputy Riek Machar, a Nuer, who was sacked from his post as vice president earlier that year. Machar became a rebel leader when civil war erupted, but the African Union report disputes that he led a coup attempt. Machar and Kiir signed a peace agreement in August to end the 20-month civil war, but fighting persists in the northeastern African nation.

Amid the mass murders, perpetrators described in the report as government forces or their allies tortured their victims by forcing them to eat human flesh or jump in fires, among other things. The killings were "an organized military operation that could not have been successful without concerted efforts from various actors in the military and government circles," the report said, according to AP. "Roadblocks or checkpoints were established all around Juba and house-to-house searches were undertaken by security forces. During this operation, male Nuers were targeted, identified and killed on the spot or gathered in one place and killed."

Machar fled the capital city and mobilized an insurgency that carried out revenge attacks against the Dinka in South Sudan. The attacks sparked cycle of violence in various towns, including rape and murder of people in churches and hospitals, the report said.

Nearly 4 million people are facing famine, with thousands of others at risk of starvation, in South Sudan. The United Nations has accused the government and rebel fighters of committing atrocities and “crimes against humanity.” Aid agencies have called the conflict one of the worst humanitarian crises in the world, according to BBC News.