The attack took place on Sunday in the adjoining province of Mai-Ndombe, where according to Human Rights Watch (HRW) at least 300 people have died and more than 50,000 have been displaced since last June.
A security official confirmed the toll of nine dead.
Six bodies were taken to a morgue in Ndjili hospital in eastern Kinshasa, but "three others were in pieces... and could not be put back together," added Jean Bedel Mulele, the traditional chief in the nearby village of Kie.
The conflict opposes the Teke, who consider themselves the historic settlers of a swathe of around 200 kilometers of land on the bank of the Congo River, and the Yaka, who the Teke say arrived after them.
An armed group carried out the pre-dawn attack on the village of Yosso, "killing nine people, including two children," according to Reagan Mangawana, who hails from the village and said he went there from his home in Kinshasa on Monday.
Five people were seriously hurt and the southern part of the village was completely burned down, he told AFP late Monday.
"It was the Mobondo who attacked," he said, using a name for a militia of the Yaka community.
The festering conflict in Mai-Ndombe has been largely overlooked outside the Democratic Republic of Congo, as neighboring countries focus on decades-long problems in the country's east.
