Suspected Ugandan rebels killed at least 15 Tanzanian U.N. peacekeepers and wounded 53 others in a raid on a base in Congo that U.N. Secretary-General Antonio Guterres on Friday called the worst attack on the organisation in recent history.

Tanzania’s President John Magufuli said he was “shocked and saddened” by the deaths, which come amid rising violence against civilians, the army and U.N. troops in Democratic Republic of Congo’s eastern borderlands.

The U.N. chief said the attack constituted a war crime and called on Congolese authorities to investigate and “swiftly bring the perpetrators to justice”.

“I want to express my outrage and utter heartbreak at last night’s attack,” Guterres told reporters at U.N. headquarters in New York. “There must be no impunity for such assaults, here or anywhere else.”