At least 890 people are believed to have been killed in ethnic violence in northwestern Democratic Republic of the Congo (DRC) last month, the United Nations human rights office said on Wednesday, warning that the actual 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 been killed in bloodshed that led the government to cancel voting there in last month's presidential election.

"I have to emphasise that 890 is the number of people we know were actually buried," UN 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, the UN rights office said.

"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 DRC, in what appear to have been clashes between the Banunu and Batende communities," the UN human rights office statement read.

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 election, a local activist told the Reuters news agency 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.

Voting was cancelled in three cities in the area during the December 30 presidential polls. The election was meant to lead to the DRC's first democratic transfer of power since it gained independence from Belgium in 1960 but the provisional results have been disputed by opposition groups and monitors.

"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 in a statement.

The rights office said Wednesday that it had launched an investigation into the reports of violence. National judicial authorities are also conducting a probe, according to the office.

Investigating the claims "is essential to ensure justice for the victims of these horrific attacks, but also to prevent new episodes of intercommunal strife, and to address the anger and feelings of gross injustice that may otherwise lead to repeated cycles of violence between communities," Bachelet said.