Rebels raided a site outside the eastern town of Fataki where hundreds of civilians have sought refuge in recent months, killing 14 people including children, army spokesman Jules Ngongo Tsikudi said.
Civil society leader Dieudonne Lossa gave a provisional death toll of 15 and blamed a militant group known as CODECO, accused of staging another attack on a nearby artisanal mining site on Sunday that killed at least 35.
Reuters was not able to reach CODECO for comment on Tuesday.
The group is one of several armed militias, including an Islamic State affiliate, wrangling over land and resources in Congo's mineral-rich east - a conflict that has claimed thousands of lives and displaced millions over the past decade.
Congo's government declared martial law in Ituri and neighbouring North Kivu province a year ago in an effort to quell the violence. But deadly raids have surged since then.
CODECO is renowned for targeting civilians, killing 18 people at a church last month and another 60 at a displaced persons camp in February.