Fighters believed to be from the Allied Democratic Forces (ADF) killed residents and burned down houses in Kandoyi and Bandiboli villages in Ituri province late on Friday and early on Saturday, said Christophe Munyanderu, coordinator of the local group Convention for the Respect of Human Rights (CRDH).
The Congolese army spokesperson in Ituri, Jules Ngongo, confirmed about 20 deaths, and said Congolese forces were in pursuit of the assailants.
"It's too hard for me - when I saw the bodies, their throats had been cut," said Alice Kyanga, whose parents were among those killed at their homes on Saturday.
The ADF is a Ugandan militia that moved to eastern Congo in the 1990s. The group was responsible for 1,050 violent deaths in 2021, up from 599 in 2020, according to data from Kivu Security Tracker, which maps violence in the area.
Security has deteriorated in Ituri and neighbouring North Kivu province since the government placed them under military rule last year, U.N. experts said in June.
The government declared a state of siege in the provinces in April 2021 in response to escalating attacks by militias, including repeated massacres by the ADF, which has links to Islamic State.
Uganda has also sent at least 1,700 troops to help its neighbour fight the ADF after accusing the group of responsibility for a string of bombings in Kampala last year.