At least 890 people were killed during inter-communal clashes in four villages in western Democratic Republic of Congo over just three days last month, the United Nations said Wednesday.
"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 UN rights office said it had been informed by "credible sources" that at least 890 people were killed between December 16 and 18 in four villages in Yumbi territory, in Mai-Ndombe province, "in what appear to have been clashes between the Banunu and Batende communities".
It said that at least 82 people were also reportedly injured in the attacks, but that the actual number of casualties was expected to be higher.
The violence appears to have been sparked when members of the Banunu tribe wanted to bury one of their traditional chiefs on Batende land.
Around 465 houses and building were burned down or pillaged, including two primary schools, a health centre, a market and the office of the national elections commission, it said.
The UN refugee agency said earlier this month that some 16,000 people had fled from the affected villages and into the neighbouring Republic of Congo, also known as Congo-Brazzaville.
That agency said the violence was not linked to recent elections in the DRC, but that it had instead been caused by the resurgence of an old rivalry between Banunus and Batende communities.
In 2009, ethnic clashes in the region forced some 130,000 people to seek shelter in Congo -- which currently hosts some 60,000 refugees, mainly from the DRC, the Central African Republic and Rwanda.
The rights office said Wednesday that it had launched an investigation into the reports of violence.
National judicial authorities are also conducting a probe, it said.
Investigating the claims "is essential to ensure justice for the victims of these horrific attacks, but also to prevent new episodes of inter-communal strife, and to address the anger and feelings of gross injustice that may otherwise lead to repeated cycles of violence between communities," Bachelet said.
The UN rights chief said her office could provide advice and support "in the conduct of investigations, as well as in efforts to prevent the recurrence of such violence, and to work towards justice and reconciliation."