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KINSHASA, Congo — 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."

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

U.N. peacekeepers in the Democratic Republic of Congo in 2016. Eduardo Soteras / AFP/Getty Images file

The peacekeeping base is about 45 kilometers (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.

Members of the U.N. Security Council, which authorized the peacekeeping mission, stood in silent tribute to the victims at the start of a meeting Friday afternoon on a five-nation force in Africa's Sahel region.

Later, the council condemned the attack "in the strongest terms," underlined that deliberate attacks targeting peacekeepers may constitute war crimes, and called on Congo's government to ensure that the perpetrators "are swiftly brought to justice."

"This attack, the worst on U.N. peacekeepers in recent years, is a reminder of the extraordinary sacrifices made by these brave women and men every day," the council said.

Birnback said U.N. flags will fly at half-staff Monday all over the world in memory of those killed or injured.

Congo, the size of Western Europe, has seen immeasurable cruelty and greed as a result of its mineral resources while more than 80 percent of the population lives below the absolute poverty line of $1.25 a day. The nation suffered through one of the most brutal colonial reigns ever known before enduring decades of corrupt dictatorship. Back-to-back civil wars later drew in a number of neighboring countries.

Many rebel groups have come and gone during the U.N. mission's years of operation, at times invading the regional capital.

One of the greatest threats now comes from the ADF. While the fighters are mainly Muslim, experts say there are no proven links between the rebels and other extremist organizations in Africa.

The ADF "has an agenda both ideological and extremist in nature, but also focused ... on exploitation of illegal resources," Lacroix said. He accused the rebels of preying on the local population.

The rebels once aimed to overthrow President Yoweri Museveni's regime in neighboring Uganda. By the 1990s, they had established themselves in Congo.

Human rights groups say at least 1,000 people have been killed in the last three years as the ADF intensified attacks in Congo. About a dozen rebels have been sentenced to death on charges related to participating in an insurrection movement.

The U.N. mission in 2006 helped carry out Congo's first free and fair elections in 46 years. Since then, the winner of that vote, President Joseph Kabila, has become entrenched in power. Anger has grown as presidential elections originally set for late last year have been repeatedly delayed. The government has blamed insecurity for the delays.

The U.N. secretary-general in October warned that without real progress toward elections, Congo "is likely to enter a period of extreme volatility."