Photo: The Canadian Press

FILE - In this Friday Nov. 30, 2012 file photo, Two UN soldiers stand guard in Goma, Democratic Republic of Congo. Rebels attacked a United Nations peacekeeping base in eastern Congo, killing at least 14 peacekeepers and wounding 40 others Friday Dec. 8 2017, in the worst violence against the mission in this Central African country in years. Deputy spokesman Farhan Haq in New York said the peacekeepers were mainly from Tanzania, and that at least five Congolese soldiers also were killed in the assault blamed on one of the region's deadliest rebel groups. (AP Photo/Jerome Delay File)

In the deadliest single attack on a United Nations peacekeeping mission in recent memory, rebels in eastern Congo killed at least 14 peacekeepers and wounded 53 others in an assault on their base that was launched at nightfall and went on for hours.

U.N. Secretary-General Antonio Guterres expressed "outrage and utter heartbreak" and called the attack a war crime, urging Congolese authorities to swiftly investigate. The State Department's Bureau of African Affairs said it was "horrified."

The peacekeepers killed were from Tanzania. At least five Congolese soldiers also were killed in the attack Thursday evening that has been blamed on one of the region's deadliest rebel groups.

Three peacekeepers remained missing, the U.N. said. More than 20 were evacuated for medical treatment in the regional capital, Goma.