The CODECO insurgents killed at least 17 people captured in the Djugu territory, around 45 kilometres (30 miles) north of Bunia in Ituri province, local community leader Banguneni Gbalande says.
Gbalande said he had been alerted by the families of some of those killed.
CODECO, the Cooperative for the Development of the Congo, is one of myriad armed groups operating in the restive, mineral-rich region.
The people were taken hostage after three CODECO members were killed in a clash with a rival militia, said one local resident in Bambu, one of the two villages attacked Saturday.
Among those taken hostage was a pregnant woman, said the source, who did not want to be identified for security reasons.
Since the end of 2022, dozens of people have been killed in gold-rich Ituri province every week. In recent days, CODECO fighters have been blamed for a series of massacres, claiming he lives of 30 people, including women and children.