Congolese authorities in Uvira say the rebel M23 group is responsible for two mass graves identified in the South Kivu city.
M23 seized control of Uvira in early December but withdrew last month.
International NGO Human Rights Watch previously claimed that M23 fighters harassed and intimidated civilians during their occupation. It also cited cases of murder, disappearance and forced recruitment, and reported that about 10 people had been killed.
M23 has denied the accusations.
According to local authorities, two graves were found in Kavinvira municipality. One site contains six bodies, local residents said, while another contains several dozen.
Residents of Sange, some 30 kilometers outside Uvira, also reported a grave containing 10 bodies.
No bodies have yet been exhumed and none of the sites have been excavated.
