In a document summarizing abuses committed in the DRC last year, the UN Joint Human Rights Office said the M23 had executed at least 171 civilians in the settlements of Kishishe and Bambo, in eastern North Kivu province.
A preliminary UN probe initially found that 131 civilians had been killed.
Human Rights Watch said in a report that the M23 had executed at least 22 people in Kishishe and killed another 10 while searching for enemy militia members.
The massacre provoked outrage in the DRC, where the Tutsi-led M23 has captured swathes of territory in North Kivu since late 2021 and displaced hundreds of thousands of people.
Reported figures for the scale of the massacre vary widely.
The DRC's government initially said that some 300 had been killed, for example, while the M23 said that eight civilians were killed by stray bullets.
Elsewhere in its statement on Tuesday, the UN noted that it had recorded nearly 6,000 human-rights violations in the DRC last year -- marking a 15-percent reduction compared to 2021.
Abuses committed by state forces also fell, the UN said, with 2,400 recorded cases last year compared to 3,162 in 2021.
Armed groups committed about 60 percent of the recorded abuses. About 85 percent of the total number of violations occurred in four provinces in the DRC's volatile east.
Despite the overall drop in recorded rights violations, there had been a "substantial increase" in the number of summary executions, the UN said.
Although it did not specify a figure, it attributed the rise to an uptick on attacks on civilians in Ituri and North Kivu provinces.
The UN pointed to the M23, Codeco, Nyatura and Allied Democratic Forces armed groups as being responsible for the trend.
