A UN investigating team said dissident forces had attacked the village of Buramba on 17 December targeting civilians suspected of sympathising with pro-government militiamen.
The probe reported that the renegade soldiers were believed to have carried out "summary and systematic executions" of civilians, including women and children, the UN mission in the Democratic Republic of Congo said in a statement.
Buramba is in North Kivu province where fighting broke out last month between former Rwandan-backed rebel group RCD-Goma and government soldiers sent to the region to reinforce the border with Rwanda. The RCD-Goma is supposed to be part of Congo's new army.
Shaky peace process
The fighting was the latest blow to a shaky peace process meant to draw a line under a five-year civil war, in which nearly four million people are estimated to have been killed.
The UN said the final death toll from the massacre may be much higher, as the soldiers were suspected to have burned corpses and killed more people in the hills overlooking Buramba. Widespread looting followed the killings.
The investigating team also heard reports that a 12-year-old girl had been raped, but was not able to confirm them.
The UN said the investigation
team was intimidated by rebels
In Buramba, renegade soldiers from the RCD-Goma former rebel group clashed last month with Mai Mai militiamen, traditional warriors loyal to the Kinshasa government.
The killing by Mai Mai fighters of three RCD-Goma soldiers who were trying to steal rice prompted the reprisal attack, the UN statement said.
The UN team had found Buramba - whose population was estimated at 10,000 - completely deserted nearly two weeks after the attack.
It said the probe was conducted in an atmosphere of insecurity and intimidation, with RCD-Goma soldiers trying to cover up what had happened and still harassing the population.
Local authorities had denied that civilians had been executed, blaming the deaths on stray bullets during clashes with Mai Mai fighters, the UN said.
However, the UN said the Mai Mai had already left Buramba when the reprisal attack was carried out.