Ethnic violence in western Democratic Republic of Congo left at least 890 dead over just three days last month, the UN says.
The UN human rights office quoted "credible sources" saying clashes between Banunu and Batende communities took place in four villages in Yumbi.
Most of the area's population has reportedly been displaced.
UN spokesperson Ravina Shamdasani said it was feared the death toll was actually much higher.
"I have to emphasise that 890 is the number of people that we know were actually buried but there are reports that many others may have been killed.
"Their bodies may have been dumped in the Congo River or they may have burned to death."
Voting in the 30 December presidential election was postponed in Yumbi because of violence.
More than 460 houses and buildings were burned down or pillaged, in the attacks, which reportedly took place on 16-18 December.
They included 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 about 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," UN High Commissioner for Human Rights Michelle Bachelet said.
At least 82 people were 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 suggested 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.
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.