At least 890 people are believed to have been killed over three days in December in ethnic violence in northwestern Democratic Republic of Congo, the United Nations said on Wednesday, and it warned that the toll could be higher still.

This doubles an estimate provided on Monday by a local priest and a civil society activist who said that at least 400 people had died in bloodshed that led the government to cancel voting there in last month's presidential election.

The fighting between the Banunu and Batende communities in Mai-Ndombe province was some of the worst in the area for years and highlighted the precarious state of inter-ethnic relations even in the Central African country's more peaceful regions.

"I have to emphasise that 890 is the number of people we know were actually buried," U.N. human rights spokeswoman Ravina Shamdasani said after the release of a statement on the attacks.

"But there are reports that many others may have been killed and their bodies may have been dumped in the Congo River or they may have been burned to death," she said.

Communal fighting and widespread pillaging around the town of Yumbi led to an estimated 16,000 people seeking refuge by crossing the Congo River into the Republic of Congo, according to the U.N. rights office statement.

It said 465 houses and buildings, including schools, a health centre, market and office of the national electoral commission, had been burned or pillaged.

The violence broke out over a dispute linked to a tribal chief's burial, Shamdasani said.

While the bloodshed was not directly related to the end-of-year election, a local activist told Reuters in December that tensions between the two ethnic groups had festered because Batende leaders were supporting the ruling coalition while Banunu leaders backed opposition candidates.

The Dec. 30 vote was meant to lead to Congo's first democratic handover of power since independence from Belgium in 1960. But hope for a new era after 18 years of chaotic rule by President Joseph Kabila has faded in a welter of controversy.

The second-place finisher, Martin Fayulu, has contested the result based on his camp's tallies, saying that the official winner, opposition leader Felix Tshisekedi, struck a deal with Kabila. Tshisekedi and Kabila deny this.