KINSHASA, Congo (AP) — In the deadliest single attack on a United Nations peacekeeping mission in nearly 25 years, rebels in eastern Congo killed 15 peacekeepers and wounded over 50 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.”

U.N. peacekeeping spokesman Nick Birnback said it was the deadliest attack on a U.N. peacekeeping mission since June 1993, when 22 Pakistani soldiers were killed in Somalia’s capital, Mogadishu.

The peacekeepers killed Thursday were from Tanzania. Tanzanian President John Magufuli expressed his shock and prayers for the wounded, three of whom are in critical condition. 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 were missing, the U.N. said. More than 20 were evacuated for medical treatment in the regional capital, Goma.

Birnback, the U.N. peacekeeping spokesman, called the assault “a determined and well-coordinated attack by a well-armed group.”

It was not clear when military reinforcements arrived after the attack, the U.N. said. Conditions in the region are “very, very challenging,” said U.N. peacekeeping chief Jean-Pierre Lacroix, who said the attack followed a recent increase in activities by various armed groups. He called the assault a response to the U.N. mission’s own “increasingly robust posture.”

“We are disturbing them,” he said. “They do not like it.”

The peacekeeping base is about 27 miles from the town of Beni, which has been repeatedly attacked by the Allied Democratic Forces, or ADF, rebel group. The ADF is suspected of being behind Thursday’s assault. The base is home to the U.N. mission’s rapid intervention force, which has a rare mandate to go on the offensive against armed groups in the vast, mineral-rich region.

The U.N. mission in Congo is the largest and most expensive in the world. It has also been a high-profile target of the Trump administration’s cost-cutting efforts. The mission has a budget of $1.14 billion and over 16,500 soldiers. Nearly 300 peacekeepers have been killed since the mission arrived in 1999, according to U.N. data.