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.

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

UN peacekeeping spokesman Nick Birnback said it was the deadliest attack on a UN 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 UN said. More than 20 were relocated for medical treatment in the regional capital, Goma.

Birnback, the UN peacekeeping spokesman, called the assault "a determined and well co-ordinated attack by a well-armed group."

It was not clear when military reinforcements arrived after the attack, the UN said. Conditions in the region are "very, very challenging," said UN 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 UN mission's own "increasingly robust posture."

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

The deadly attack on peacekeepers occurred in North Kivu province, the UN said. (CBC)

Rebel group suspected

The peacekeeping base is about 45 kilometres 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 UN 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 UN 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 UN data.

Two UN soldiers stand guard in Goma, in the Democratic Republic of Congo in November 2012. (Jerome Delay/Associated Press)

Members of the UN 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. Birnback said UN 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 per cent 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 neighbouring countries.

No new election date

Many rebel groups have come and gone during the UN 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 neighbouring 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 UN 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 UN secretary-general in October warned that without real progress toward elections, Congo "is likely to enter a period of extreme volatility."