Another 38 probable mass graves have been found in the Democratic Republic of Congo, where violence between troops and armed fighters has killed thousands of people since August, the United Nations announced on Wednesday.

This means at least 80 mass graves have been identified so far, the UN peacekeeping mission in the vast Central African nation said.

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 the Congo's military justice authorities, the UN said.

The international community has expressed alarm over the violence in the once-calm Kasai province region. Some diplomats have suggested that the tensions are also tied to Congo's presidential election that has been delayed since last year.

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The Kasai crisis was sparked by the government's refusal to grant official status to Jean-Pierre Mpandi, known as Kamuina Nsapu, as one of the region's traditional chiefs.

Mpandi then mobilised his armed group in an uprising against the state's presence in the area. After he was killed during fighting in August 2016, the violence escalated.

More than 3,000 people have been killed, nearly 1.3 million displaced and about 30,000 have fled to neighbouring Angola, according to the Catholic Church and the UN.

The government has accused the Kamuina Nsapu armed group of dumping bodies into mass graves. But UN investigators believe many of the mass graves contain the remains of suspected Kamuina Nsapu members or supporters.

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".

The Congo's government now points to the violence as the reason for further election delays. The country's ambassador to the UN, Ignace Gata Mavita, 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 July 20, he said.

President Joseph Kabila's mandate ended in December, and after deadly protests, the government and opposition reached a December 31 agreement that calls for the vote by the end of this year - without Kabila as a candidate.

The head of the electoral commission, however, now says that timeframe won't be possible.