The group said 141 people, predominantly ethnic Hutus, were feared dead or missing after the attacks near Virunga National Park in North Kivu province, citing local experts and witness accounts.
It said the killings appeared to be part of a military campaign by the M23 group, the most prominent of more than 100 armed groups in mineral-rich eastern Congo, against the Democratic Forces for the Liberation of Rwanda (FDLR), a mostly Hutu armed group.
Nearly 2 million Hutus from Rwanda fled to Congo after the 1994 Rwandan genocide that killed 800,000 Tutsi, moderate Hutus and others. Rwandan authorities have accused Hutus who fled of participating in the genocide, alleging that the Congolese army protected them.
The United States and others have been trying to achieve a permanent ceasefire since fighting between the M23 and Congolese forces escalated in January with the M23’s seizure of two key cities. U.S. President Donald Trump this week asserted he had “ended six wars” including this one, but experts say his impact isn’t as clear cut as he claims.