A United Nations rights group has unearthed more than 50 mass graves in the western Democratic Republic of Congo after a spate of killings were reported in the region last month.

Abdoul Aziz Thioye, the director of the UN Joint Human Rights Office (UNJHRO), said on Saturday that "more than 50 mass graves, as well as common and individual graves that we have identified," were uncovered in the town of Yumbi in the DRC's western province of Mai-Ndombe.

"This suggests that the number [of deaths] is quite high because a communal grave depending on size may contain five, ten bodies" or even "one hundred bodies or four times more," Thioye said following a joint fact-finding mission with local authorities.

General Fall Sikabwe, the army chief in western DRC, told the AFP news agency that an investigation had begun.

"They have killed soldiers and policemen, taking their weapons to slaughter with," he said, giving no further details about the killings.

Earlier this month, the UN said at least 890 people were killed during three days of inter-communal clashes in the region.

UN High Commissioner for Human Rights Michelle Bachelet, said in a statement on January 16 that the UN office had been informed by "credible sources" that the people were killed between December 16 and 18 in four villages in Yumbi.

The violence appears to have been rooted in a long-standing rivalry between the Banunu and Batende ethnic groups, sparked when Banunu tribespeople buried a traditional chief on Batende land on the night of December 13.

Around 465 houses and building were then burned down or pillaged, including two primary schools, a health centre, a market and the office of the national elections commission, the UN rights office said.

The UN refugee agency said earlier this month that 16,000 people had fled from the villages into the neighbouring Republic of Congo, also known as Congo-Brazzaville.

In 2009, ethnic clashes in the region forced 130,000 people to seek shelter in the Republic of Congo - which now hosts 60,000 refugees, mainly from the DRC, the Central African Republic and Rwanda.