“I want to express my outrage and utter heartbreak at last night’s attack on United Nations peacekeepers in the Democratic Republic of Congo,” Mr. Guterres said. “It is another indication of the enormous sacrifices made by troop-contributing countries in the service of global peace.”
Peacekeeping officials said the North Kivu attack constituted the worst single loss since 22 Pakistani peacekeepers were killed in Somalia in 1993.
The United Nations declined to say how many militants were killed in the North Kivu attack, but a spokesman for the Congolese Army in North Kivu, Capt. Mak Hazukay, said 72 died.
The raid took place at a place called Semuliki, on the border with Uganda. The region was a stronghold for the militant group, according to the Congo Research Group, but in early 2014, most of its camps there were destroyed in raids by the Congolese Army, with the support of United Nations peacekeepers.
Hundreds of civilians were killed in the region in the years following the raid, and the United Nations has blamed the rebel group for much of the killing.
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The mission’s statement provided few details about how the attack unfolded, but it said that the United Nations and Congolese forces had been coordinating a joint response and the agency had been conducting evacuations and assessing the security situation in the area.
The United Nations Organization Stabilization Mission for the Democratic Republic of Congo, known by its acronym, Monusco, is the largest United Nations peacekeeping mission in the world, with more than 16,000 soldiers.
The force was deployed to Congo in 2000 to monitor a five-nation peace deal that was mean to end foreign hostilities in the country. But militias with varying political and economic objectives have continued to destabilize the country, especially the eastern provinces. Some of those militias are made up foreign fighters, and some local armed groups have enjoyed support from foreign governments, according to United Nations reports.
Jean-Pierre Lacroix, the undersecretary for peacekeeping operations, told a news conference at the United Nations that the peacekeepers had been part of what is known as the Force Intervention Brigade, whose members have the authority to shoot first and not wait to fire in self-defense.
The Allied Democratic Forces was one of many armed groups that Monusco has been fighting under its mandate to protect civilians, he said. “We are disturbing them, and they do not like it,” Mr. Lacroix said. “They are preying on the population.”
Mr. Lacroix also said the group is “ideological and extremist in nature” but that much more needs to be learned about it. “We are trying to have a better knowledge of the A.D.F.,” he told reporters.
He said that he would travel to Congo in the coming days, and that the United Nations was “determined to get to the bottom” of what had happened.
By the end of 2016, 240 United Nations personnel had died in attacks over five years, according to the United Nations. Last year, 32 United Nations or United Nations-linked personnel were killed, 26 of them peacekeepers, predominantly on missions in Africa. The highest fatalities in individual attacks numbered six and five, both in Mali.
In October, the Allied Democratic Forces killed 22 people, many of them state officials, in the Beni region, when they ambushed a convoy of motorcycles with machetes and guns, according to the authorities.