"The provisional toll is 34 civilians killed," said local official Joy Bokele, referring to an attack by a Nande ethnic militia on a displaced persons camp in the Hutu village of Luhanga.
"They started by attacking the FARDC (DR Congo military) position. While they were attacking the FARDC, another group was executing the population with bladed weapons or bullets," Bokele said, according to AFP.
Tensions between the Nande and Hutu peoples have been running high in the restive east, shaken by two decades of fighting over land, ethnic tensions and mineral riches.
Sunday's killings are the worst inter-communal violence in a year. Dozens of people have died since the start of the year in fighting between the Nande and the Hutus.
The UN mission in the Democratic Republic of Congo, or MONUSCO, said a peacekeeping force in the area had heard gunshots and were "quickly deployed and neutralized the attack", killing one of the attackers, adding that 15 people had been wounded and now evacuated.