The chairman of a commission inquiry into the conflict in South Sudan says the African Union has postponed releasing a report which is expected to reveal those responsible for atrocities committed in the violence.

Former Nigerian President Olesugun Obasanjo said Friday the African Union Peace and Security Council had deferred releasing the report to advance a mediation process that is about "to achieve the formation of an interim government of national unity." Obasanjo spoke at the two-day African Union summit in the Ethiopian capital.

Rights groups have criticized the African Union for not releasing the report saying it would have helped bring accountability.

Fighting broke out December 2013 between President Salva Kiir's troops and those loyal to Riek Machar, who is Kiir's former vice president.