At least 15 United Nations peacekeepers and five members of the Armed Forces of the Democratic Republic of the Congo were killed in an attack by militants in North Kivu province, the organisation said on Friday. Over 50 peacekeepers were injured in the attack.

UN Secretary-General Antonio Guterres described it as the “worst attack” on its peacekeepers in recent history. “These deliberate attacks against UN peacekeepers are unacceptable and constitute a war crime,” Guterres said in a statement. “There must be no impunity for such assaults, here or anywhere else.”

The attackers, who were armed with mortars and rocket-propelled grenades, destroyed personnel carriers, an ambulance and a truck, The Guardian reported. UN troops were conducting search operations for three peacekeepers who had gone missing during the firefight.

Although no militant organisation has claimed responsibility for the attack, UN officials believe the attackers belonged to the rebel group Allied Democratic Forces, Reuters reported.