Another 38 probable mass graves have been found in central Congo, where violence between troops and militia members has killed thousands of people since August, the UN said.
This means at least 80 mass graves have been identified so far, the UN peacekeeping mission in the vast Central African nation said.
The international community has expressed alarm over the violence in the once-calm Kasai provinces region.
Some diplomats have suggested that the tensions are also tied to Congo's presidential election that has been delayed since last year.
The Catholic Church has estimated more than 3,300 people have died in the fighting since a traditional chief was killed in a military operation in August.
The UN says more than a million people have been displaced, making Congo the African country with the most internally displaced people - 3.8 million.
The latest mass graves were found this month in the Diboko and Sumbula areas of Kamonia territory by an investigative team from the local UN human rights office and Congo's military justice authorities, the UN said.
On Tuesday, UN peacekeeping chief Jean-Pierre Lacroix expressed serious concern to the UN Security Council that violence in the Kasai provinces "has reached very disturbing levels".
Congo's government now points to the violence as the reason for further election delays.
The country's UN ambassador Ignace Gata Mavita has said voter registration has not yet begun in two provinces - Kasai and Kasai Central - as a result of the fighting.
Registration is scheduled to begin on July 20, he said.
President Joseph Kabila's mandate ended in December, and after deadly protests the government and opposition reached an agreement on December 31 that calls for the vote by the end of this year - without Mr Kabila as a candidate.
The head of the electoral commission, however, now says that will not be possible.