Rebel groups, including one with ties to the Islamic State group, killed at least 21 people in restive northeastern Congo over the weekend, local officials said Monday.

Isaac Kibira, deputy delegate of the governor in the Central African nation’s northeastern Bwito township, said the victims in one attack were shot and killed early Sunday by members of M23, a rebel group the United Nations says has links to neighboring Rwanda. Rwanda denies the accusation.

In a second attack on Saturday, armed rebels with ties to the Islamic State group killed 10 civilians in a northeastern province, a local human rights activist said.

Conflict has simmered in eastern Congo for decades as more than 120 armed groups fight for control of valuable mineral resources and some to protect their communities. Mass killings by rebel groups are frequent, and the violence has triggered an exodus of refugees.