U.N. officials said 53 peacekeepers were also 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 has also turned peacekeepers into frequent rebel targets.
United Nations 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. Having organized in the mid-1990s, it operates primarily in the mountains marking Congo's border with Uganda. The United Nation'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 in the region has spiked with clashes involving rebels and security forces as well as inter-communal fighting. Analysts say there are from 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," said the Norwegian Refugee Council's (NRC) country director in Congo, Ulrika Blom, in a report earlier this week.
In 2013, U.N. peacekeepers helped weaken the largest armed group in eastern Congo, called the 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 U.N. official.
More than 300 U.N. personnel have been killed in Congo since 2001, according to UN 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 counterterror 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 spokesperson for U.S. Africa Command said Friday there are fewer than 10 Defense Department personnel there, mostly at the embassy in Kinshasa.
- The Washington Post's Alex Horton and Anne Gearan in Washington contributed to this report
