Suspected Ugandan rebels killed at least 15 Tanzanian UN peacekeepers and wounded 53 others in a raid on a base in Congo that UN Secretary-General Antonio Guterres 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 UN troops in Democratic Republic of Congo's eastern borderlands.
The UN 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 UN headquarters in New York. "There must be no impunity for such assaults, here or anywhere else."
The United Nations Security Council condemned the attack and held a moment of silence for the victims.
UN troops were still searching for three peacekeepers who went missing during the more than three-hour firefight that broke out at dusk on Thursday evening, Ian Sinclair, the director of the UN Operations and Crisis Centre, said.
UN officials said they suspected militants from the Allied Democratic Forces (ADF) staged the assault on the base in the town of Semuliki in North Kivu's Beni territory.
The ADF is an Islamist rebel group that has been active in the area. Congo's UN mission, MONUSCO, said it was coordinating a joint response with the Congolese army and evacuating wounded from the base.
Five Congolese soldiers were also killed in the raid, MONUSCO said in a statement. Congo's army said only one of its soldiers was missing, while another had been injured, adding that 72 militants had been killed.
The area has been the scene of repeated massacres and at least 26 people died in an ambush in October.
The government and UN mission blamed almost all the violence on the ADF but UN experts and independent analysts say other militia and elements of Congo's own army have also been involved.
Thursday's raid was the third attack on a UN base in eastern Congo in recent months.
Established in 2010, MONUSCO is the United Nations' largest peacekeeping mission and had recorded 93 fatalities of military, police and civilian personnel.
