United Nations (UN) investigators have discovered 17 new mass grave sites in central Democratic Republic of Congo, bringing the total to 40 documented in an area where the army has clashed with a local militia, the UN said on Wednesday.

A UN statement said the grave sites were reportedly dug by Congolese soldiers after fighting with the Kamuina Nsapu militia in Kasai Central province last month.

“At least 74 people, including 30 children, were reported to have been killed by soldiers as a result of these clashes,” the statement said.

A government spokesman did not immediately respond to a request for comment.

The government has previously denied that soldiers have used disproportionate force against militia members and said the militia had dug the graves.

The Kamuina Nsapu uprising has become the most serious threat to President Joseph Kabila, whose decision to stay in power after his mandate ran out in December stoked lawlessness in the vast central African nation.

More than 400 people have been killed in fighting in Kasai Central since last August, when Congolese forces killed the tribal militia's leader Kamuina Nsapu, escalating the conflict.