"The victims were arbitrarily executed with bullets or bladed weapons," the U.N. said in a statement.
A preliminary investigation by the U.N. peacekeeping mission (MONUSCO) and the Joint Human Rights Office (UNJHRO) into Nov. 29-30 massacres in the villages of Kishishe and Bambo found they were undertaken in retaliation for clashes between M23 and rival armed groups.
Investigators interviewed 52 victims and direct witnesses, and various other sources in Rwindi, about 20 kilometers from Kishishe, where survivors and witnesses had taken refuge, the U.N. said.
"MONUSCO condemns in the strongest terms the unspeakable violence against civilians and calls for unrestricted access to the scene and the victims for emergency humanitarian assistance," the statement said.
The government has accused M23 of killing as many as 272 people. The militia, which has captured several towns near the borders of Rwanda and Uganda this year, has denied responsibility and asked for a full investigation.
"We gave our version of the facts. We asked that there be investigations together with us in Kishishe but the U.N. never came," M23 spokesman Lawrence Kanyuka told Reuters.
"The U.N. is under pressure from the government to come up with a figure, even if it is false," he said.
The rebel group says it is ready to withdraw from occupied territory and will support regional peace-making efforts, despite not being represented in the talks, the third round of which concluded without resolution in Kenya's capital, Nairobi, this week.