Ethnic violence in western Democratic Republic of Congo left at least 890 dead over just three days last month, the UN says.
"Credible sources" say clashes between Banunu and Batende communities took place in four villages in Yumbi, the UN Human Rights Office says.
Most of the area's population has reportedly been displaced.
Voting in the 30 December presidential election was postponed in Yumbi because of violence.
The attacks had reportedly taken place on 16-18 December.
What's the UN saying?
Some 465 houses and buildings were burned down or pillaged, including two primary schools, a health centre, a health post, a market and the office of the country's independent electoral commission, the UN said.
The displaced residents included some 16,000 people who sought refuge by crossing the Congo river into neighbouring Republic of Congo, also known as Congo-Brazzaville, it added.
"It is crucial that this shocking violence be promptly, thoroughly investigated and the perpetrators be brought to justice," said UN High Commissioner for Human Rights Michelle Bachelet.
At least 82 people had been injured in the attacks, according to the reports, but the UN said it expected the number of casualties to be higher.
The UN Human Rights Office said it had launched an investigation.
Yumbi, in Mai-Ndombe Province, is normally a peaceful area, correspondents say.
Reports suggest the clashes started when members of the Banunu tribe wanted to bury one of their traditional chiefs on Batende land.
Voting in the presidential election there, as well as in Beni and Butembo in eastern North Kivu Province, was postponed until March with the electoral commission blaming insecurity and an Ebola outbreak.
What's the latest on the disputed election?
Opposition candidate Felix Tshisekedi was declared winner but another opponent of the current administration, Martin Fayulu, insists he won, alleging that Mr Tshisekedi made a deal with outgoing President Joseph Kabila.
Mr Fayulu filed an appeal in the Constitutional Court on Saturday demanding a manual recount of votes.
The issue will be discussed at meetings of the African Union and the southern African regional body Sadc in the Ethiopian capital, Addis Ababa, on Thursday, AFP news agency reports.
Mr Kabila has been in office for 18 years and the result, if confirmed, would create the first orderly transfer of power since independence from Belgium in 1960.