M23 fighters at the Stade de l'Unité in Goma, Democratic Republic of Congo. AFP
The attack was in the village of Nkana, about 75 kilometres (47 miles) northeast of the capital Kinshasa, in Kwamouth territory on November 23, HRW said in a statement.
Citing witnesses, the group said Mobondo fighters, considered members of the Yaka community, carried out the killings, which included women and children. Many more people were wounded.
The assailants, armed with rifles and machetes, went from house to house, killing mostly ethnic Teke villagers as they tried to flee, HRW said.
It was "an apparent retaliatory attack linked to rising intercommunal tensions", it added.
Violence broke out in the western region in 2022 between the Teke, who consider themselves to be the owners of villages along the Congo River, and the Yaka people, who settled in the area after them.
The Mobondo are accused of playing an active part in the violence, which left hundreds dead.
Mediation attempts by Congolese authorities have failed to halt the violence.
HRW said the fighters in the November assault had accused Nkana's Teke of refusing to install a new Yaka local chief and had sent threats before the attack.
The conflict in western DRC is overshadowed by violence in the mineral-rich east, where the M23 armed group has captured large swathes of land.
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