Rebels in the Central African Republic (CAR) killed 32 civilians last December after clashes with a rival faction, Human Rights Watch (HRW) says.
The Union for Peace in the Central African Republic (UPC) is said to have rounded victims up and executed them.
HRW says it is concerned UN peacekeepers could not stop the atrocities.
UPC is a splinter group formed from the Seleka movement which briefly seized power in March 2013 after a coup.
The Seleka group was itself then ousted, leading to a wave of violent reprisals against the Muslim population by the Christian anti-Balaka militia.
UPC fighters killed 25 people in the central town of Bakala on 12 December after calling them to a school for a meeting, HRW said.
They had earlier killed seven men who were returning from a nearby gold mine, it added.
"These executions are brazen war crimes by UPC fighters who feel free to kill at will," said HRW's Lewis Mudge.
"The group is carrying out its killing sprees with no fear of punishment, despite the presence of United Nations peacekeepers."
Thousands of people have been killed and hundreds of thousands displaced in the CAR since 2013.
More than 12,000 UN peacekeepers are deployed in the country.
Their presence is credited with helping to reduce violence that at its peak in 2014 had led to fears of possible genocide.
However the country remains plagued by insecurity and has seen a resurgence of rebel factions.
UN troops have been accused of sexually abusing children and some central Africans have called for them to withdraw, saying they are failing to protect civilians.