The massacre occurred in the village of Boku, in Kwamouth, a region of the Democratic Republic of Congo's Mai-Ndombe province, which has seen fighting between the Yaka and Teke people since June.
On Monday, unidentified assailants armed with rifles attacked the Teke village, according to Nkete Mboma Butu, a civil-society representative in Kwamouth.
Twenty people died the attack, he said.
"The inhabitants defended themselves with machetes," Butu added. "There were dead on both sides".
Provincial deputy Moise Makami Muzik put the number of dead at 35, however, telling local reporters on Tuesday that the government in the capital in Kinshasa was indifferent to the conflict.
AFP was unable to independently confirm the death toll.
The conflict has left more than 180 people dead, the DRC government said last month. The UN says tens of thousands have been displaced.
Sixteen people were also killed in an attack in Mai-Ndombe last week.
Fighting originally erupted following a dispute over taxes and land, according to officials in the province.