Eight United Nations peacekeepers and 12 Congolese troops have been killed in an offensive against a militia in eastern Democratic Republic of Congo, the U.N. mission to the country said on Thursday, November 15.

One Tanzanian and seven Malawian peacekeepers were killed, the council said, raising the toll from an earlier count.

The deaths mark the biggest loss by the large U.N. force in the Democratic Republic of Congo since the rebels killed 15 troops nearly a year ago.

Malawi’s armed forces confirmed earlier that four of its soldiers with MONUSCO – a sergeant and three privates aged between 29 and 38 – had been killed on Wednesday.

The military “has lost courageous, hardworking and disciplined soldiers who were always ready to serve to ensure that peace prevails,” the Malawi Defence Force said in a statement.

It gave no further details on where or how they died.

Twelve Congolese soldiers were killed and another 10 peacekeepers injured, the Associated Press reported U.N. spokesperson Stephane Dujarric as saying on Thursday.

The joint operation was launched on Tuesday against the Allied Democratic Forces, a rebel group blamed for a series of attacks in the region.

The offensive aimed at Kididiwe, about 20 km from the city of Beni, said General Bernard Commins, deputy head of the U.N. peacekeeping force MONUSCO.

“We are holding Kididiwe at present, after violent fighting with an armed group. At present, we are evacuating wounded Congolese troops and Blue Helmets,” he told AFP.

Commins said he was “unable” to say if there had been fatalities, amid reports that the peacekeepers had suffered losses.

Kididiwe is a “major stronghold” of the ADF, Commins said. A Congolese officer, speaking on condition of anonymity, said the town had been used as a springboard for militia attacks on Beni’s suburbs this year.

On Wednesday, Commins said MONUSCO had deployed attack helicopters against ADF forces threatening U.N. troops in the Mayangose area, northeast of Beni.

The ADF is a militia created by Muslim rebels to oppose Ugandan President Yoweri Museveni but which also operates in the DRC.

The group has been in the east of the country since 1995 and is accused by the U.N. and Congolese authorities of committing a series of civilian massacres since 2014. They are thought to have killed at least 700 civilians and 15 Tanzanian peacekeepers.

However, a 2017 report by the Congo Research Group at New York University concluded that Congolese Army commanders were responsible for orchestrating massacres in Beni from 2014 to 2016.

The ADF has been blamed for a recent string of attacks in the region, including two raids on November 4 that killed at least seven people in the North Kivu region. Fifteen people, including children, were kidnapped in those attacks.

On Monday, MONUSCO’s head, Leila Zerrougui, sounded the alarm over violence in the Democratic Republic of Congo’s east, warning it threatened hopes for staging trouble-free elections on December 23.

MONUSCO has around 17,000 members, making it one of the U.N.’s biggest peacekeeping operations. Created in 1999, it has an annual budget of $1.153 billion (€1.02 billion).

This story was updated on November 16 at 0605 GMT to update the casualty toll.

With reporting from AFP