At least 14 UN peacekeepers and five Congolese air force members have been killed in the Democratic Republic of the Congo in the “worst attack in recent history”, according to the UN.
The UN said that at least 53 peacekeepers were also injured in the attack in North Kivu on Thursday night.
“I condemn this attack unequivocally,” Antonio Guterres, UN secretary-general, said in a statement.
“These deliberate attacks against UN peacekeepers are unacceptable and constitute a war crime.
“I call on the DRC authorities to investigate this incident and swiftly bring the perpetrators to justice. There must be no impunity for such assaults, here or elsewhere.”
The attack took place at a base in Beni, North Kivu, of the UN Organization Stabilization Mission in the Democratic Republic of Congo (MONUSCO).
The area has been repeatedly attacked by rebels of the ADF.
Farhan Haq, UN deputy spokesperson, said the peacekeepers were mainly from Tanzania.
The Associated Press news agency reported quoting a radio station in the region, which cited military sources, that the fighting lasted four hours.
Maman Sidikou, the head of MONUSCO, said the mission “will take all actions to ensure that the perpetrators are held accountable and brought to justice”.
Guterres called the incident the “worst attack” on UN peacekeepers in the “organisation’s recent history”.
Most recently, at least four peacekeepers and a Malian soldier were killed in two separate attacks in northern Mali in September.
MONUSCO took over from an earlier UN peacekeeping operation in Congo in July 2010.
Part of its mandate includes “the protection of civilians, humanitarian personnel and human rights defenders … and to support the government of the DRC in its stabilisation and peace consolidation efforts”.
Nearly 300 peacekeepers have been killed since the initial mission arrived in 1999, according to UN figures.