The toll doubles an estimate provided on Monday by a local priest and a civil society activist who said that at least 400 people had been killed in bloodshed which led the government to cancel voting there in last month's presidential election.
"According to allegations from credible sources, at least 890 people were killed between 16 and 18 December in four villages in Yumbi territory, Mai-Ndombe province in the west of the Democratic Republic of the Congo, in what appear to have been clashes between the Banunu and Batende communities," the U.N. human rights office said in a statement.
Communal fighting and widespread pillaging around the town of Yumbi, a normally peaceful area, led to an estimated 16,000 people seeking refuge by crossing the Congo River into the Republic of Congo, it added.
It was not clear if the violence was directly related to the election, the results of which are disputed. But it pointed to the kind of unrest that can quickly be unleashed.
The U.N. human rights office said 465 houses and buildings, including schools, a health centre, market and office of the national electoral commission, had been burned or pillaged.
"It is crucial that this shocking violence be promptly, thoroughly investigated and the perpetrators be brought to justice," said U.N. High Commissioner for Human Rights Michelle Bachelet.
(Reporting by Stephanie Nebehay; Editing by Angus MacSwan)