A truck operated by a U.N. peacekeeping mission is pictured near Kibumba, in the North Kivu province of Congo, on April 23, 2015. (Federico Scoppa/AFP/Getty Images)

At least 15 United Nations peacekeepers were killed and dozens wounded in eastern Congo in one of the deadliest attacks on the international forces in years, U.N. officials reported Friday.

Heavily armed rebel fighters attacked a forward operating base in a remote part of North Kivu province Thursday night, firing rocket-propelled grenades and destroying at least one armored personnel carrier, U.N. officials said. The firefight went on for at least three hours, and the majority of those killed and injured were from Tanzania, they said.

"This is the worst attack on U.N. peacekeepers in the organization's recent history," U.N. Secretary General António Guterres said in a statement. He called the assault "a war crime" and demanded that authorities in Congo — also known as the Democratic Republic of the Congo — bring the perpetrators to justice.

U.N. officials said 53 peacekeepers also were wounded in the attack, and at least five members of Congo’s military were killed.

The death toll appeared to be the highest for U.N. peacekeepers in a single incident since 1993, when 23 "blue helmets" were slain in Mogadishu, Somalia.

The U.N. peacekeeping mission in Congo, known by the acronym MONUSCO, is the largest and most expensive in the world, with roughly 19,000 peacekeepers. It has a rare mandate to pursue offensive operations against armed groups, which has resulted in some military victories but also has turned peacekeepers into frequent rebel targets.

U.N. officials said they suspected that a rebel group called the Allied Democratic Forces (ADF) was behind the attack. In the past, the United Nations has said the group has "committed serious violations of international law."

“Our sense is that given the location and given what’s happened in recent months, it was most likely the ADF,” said a senior U.N. official who spoke on the condition of anonymity, as he was not permitted to share details of the attack publicly.

The ADF is responsible for at least two other high-profile attacks on the peacekeeping force deployed in the country, one in July 2013 and another seven months later, according to a U.N. fact sheet. Since the mid-1990s, it has operated primarily in the mountains marking Congo's border with Uganda. The U.N.'s most recent head count indicated the ADF had around 1,500 fighters.

Various rebel groups and militias have battled for years in ­mineral-rich eastern Congo. In recent months, violence has spiked with clashes involving rebels and security forces as well as inter-communal fighting. Analysts say there are 30 to 60 armed groups in eastern Congo. The bloodshed has left hundreds dead and prompted a new wave of refugees to flee the country.

Meanwhile, in Congo's central province of Kasai, a humanitarian disaster is unfolding after months of clashes between local militias and security forces. The Catholic Church estimated in June that more than 3,000 people in the region had been killed since the outbreak of fighting the previous October. Earlier this year, two U.N. human rights investigators, including American Michael Sharp, were killed by militants in the province.

More than 1.7 million people have been forced from their homes in Congo this year because of insecurity, according to the United Nations.

"It's a mega-crisis. The scale of people fleeing violence is off the charts, outpacing Syria, Yemen and Iraq," Ulrika Blom, the Norwegian Refugee Council's country director in Congo, said in a report this week.

In 2013, U.N. peacekeepers helped weaken the largest armed group in eastern Congo, called the March 23 Movement, or M23, but an array of other militias continue to operate with relative impunity across the area, including the ADF.

The U.N. mission in Congo includes troops known as the Force Intervention Brigade, which frequently targets armed rebels.

“The mission has a mandate to take offensive action against armed groups, particularly the Force Intervention Brigade, and they were the ones who were attacked. These are units going after the armed groups directly,” said the senior U.N. official.

More than 300 U.N. personnel have been killed in Congo since 2001, according to U.N. records.

U.S. security assistance to Congo has focused on peacekeeping, which makes up the bulk of the $162 million in American aid spent there since 2010. The U.S. government also has invested significant resources to train and advise Congo's military and to support counternarcotics and counterterrorism initiatives.

There were 410 U.S. troops in Congo and surrounding nations in central Africa this summer, according to a June disclosure by the White House. A spokesman for U.S. Africa Command said Friday there are fewer than 10 Defense Department personnel there, mostly at the embassy in Kinshasa.

Alex Horton, Andrew deGrandpre and Anne Gearan in Washington contributed to this report.

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