JUBA, South Sudan — African Union investigators discovered mass graves in South Sudan and found evidence of horrific crimes, including forced cannibalism, according to a long-awaited report.  President Salva Kiir’s faction in the conflict is also accused of recruiting an irregular tribal force before the outbreak of war in December 2013.

The report, released late Tuesday, also disputes that there was a coup attempt in December 2013 by former Vice President Riek Machar. Government troops carried out organized killings of members of the ethnic Nuer in Juba, the capital, the report said. When violence broke out, Machar, a Nuer, became a rebel leader. He and Kiir, an ethnic Dinka, recently signed a peace agreement.

The African Union investigators, led by former Nigerian President Olusegun Obasanjo, found that the conflict began on Dec. 15, 2013, as a skirmish broke out between Dinka and Nuer soldiers of the presidential guard following political tension between Kiir and Machar, who had been fired as Kiir’s deputy the previous July.