In the deadliest single attack on a United Nations peacekeeping mission in recent memory, rebels in eastern Congo killed at least 14 peacekeepers and wounded 53 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 peacekeepers killed were from Tanzania. 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.

At least two peacekeepers remained missing, the UN said. More than 20 were evacuated for medical treatment in the regional capital, Goma.

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 peacekeeping base is about 45 kilometres from the town of Beni, which has been repeatedly attacked by the Allied Democratic Forces rebel group, which is suspected in 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, and it has been a high-profile target of the Trump administration's cost-cutting efforts. Nearly 300 peacekeepers have been killed since the mission arrived in 1999, according to UN data.

Congo, a country the size of Western Europe, has seen immeasurable cruelty and greed as a result of its mineral resources. The nation suffered through one of the most brutal colonial reigns ever known before undergoing decades of corrupt dictatorship. Back-to-back civil wars later drew in a number of neighbouring countries.

No new election date in sight

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.

The ADF intensified attacks several years ago, and human rights groups say at least 1,000 people have been killed in the last three years. About a dozen of the 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, but since then the winner of that vote, President Joseph Kabila, has become further entrenched in his post.

Anger has grown as presidential elections originally set for late last year have been repeatedly delayed, most recently to late 2018. The government has blamed insecurity as a factor in the delays.