BENI, Congo — A Congolese militia group has decapitated 42 policemen after ambushing them in an increasingly violent region where the U.N. is searching for missing American and Swedish investigators, a local official said Saturday.

Members of the Kamwina Nsapu militia staged the attack between the cities of Tshikapa and Kananga in Central Kasai province on Friday, according to Kasai Assembly President Francois Kalamba. The militia members freed six policemen because they spoke the local Tshiluba language, he said.

Large-scale violence erupted in the Kasai region in August when security forces killed the militia’s leader. More than 400 people have been killed and more than 200,000 displaced since then, according to the U.N.

The U.N. in recent days reported the discovery since January of more than two dozen mass graves in three Kasai provinces. And five videos have emerged in recent weeks that appear to show Congolese soldiers firing on militia members.

While the violence is linked to local power struggles, there are also ties to Congo’s political crisis, according to Human Rights Watch. Anger has been growing in the country at long-delayed presidential elections, and dozens were killed in December amid protests as President Joseph Kabila stayed on past the end of his mandate. A deal reached between the ruling party and opposition to hold elections by the end of this year, without Kabila, remains fragile.

Security forces have been known to back local leaders seen as loyal to Kabila, while militia groups support those who are believed to support the opposition, the rights group said.

Militia members have recruited large numbers of children and used crude weapons to attack security forces and some government buildings in Kasai, Kasai Central, Kasai Oriental, Sankuru, and Lomami provinces, Human Rights Watch said.

Parts of Congo have experienced insecurity for more than two decades since the end of the Rwandan genocide led to the presence of militias, all vying for control of mineral-rich land.