The surge in killings occurred as the Allied Democratic Forces, a Ugandan armed group that pledged allegiance to Islamic State in 2019, extended its attacks farther northward into Ituri province, the U.N. Joint Human Rights Office said.
The group often massacres civilians as retaliation for military campaigns against it.
IS has claimed responsibility for some of the ADF's violence, including a string of bombings in Uganda in October and November, and an explosion in a restaurant in the Congolese city of Beni on Christmas Day.
However, United Nations researchers say they have found no evidence of IS command and control over ADF's operations.
Congo imposed martial law in Ituri and neighbouring North Kivu province in May and began joint operations with Uganda's army in November against the ADF.
Violence levels have not come down, but Congolese authorities insist they are making progress.
Authorities on Friday detained a Kenyan ADF fighter, Salim Mohamed Rashid, government spokesman Patrick Muyaya said. He did not provide further details.
Rashid appeared in the first video of an ADF beheading last June, according to Laren Poole of the U.S.-based Bridgeway Foundation, which studies the group.
"Foreign fighters such as Salim demonstrate the reach of the ADF's networks into (East Africa) and could also pose a direct danger should the ADF decide to start sending them back to their countries of origin to establish cells, as they have done in Uganda in the past," Poole said.
In September authorities said they had arrested a Jordanian ADF fighter, who was thought to have been in charge of the militia's drones.
(Reporting by Hereward Holland; additional reporting by Fiston Mahamba and Erikas Mwisi Kambale; editing by Grant McCool)
By Hereward Holland
