Ugandan rebels fired on a United Nations helicopter in the volatile Beni region in eastern Democratic Republic of Congo, though there were no casualties, U.N. sources said.

“One of our helicopters was fired on in Semuliki valley by rebels – we reacted in legitimate defence,” French General Bernard Commins, deputy head of the U.N.’s Monusco peacekeeping force told AFP.

“The aircraft was able to land without problems but it had been hit,” added Commins.

It is unclear what weaponry was used or how badly the helicopter was damaged.

The attack came in the eastern region of Beni, where last week eight U.N. peacekeepers and 12 DRC troops died in a clash with Allied Democratic Forces near Kididiwe after they launched a joint operation targeting the ADF, a rebel group blamed for a series of attacks in the region.

The November 15 incident was the biggest loss by the large U.N. force in the Democratic Republic of Congo since the ADF killed 15 troops a year ago, in an attack also in the Semuliki area, near the Ugandan border

The ADF is a militia created by Muslim rebels to oppose Uganda’s President Yoweri Museveni but which also operates in the border area in the DRC’s North Kivu province. It is led by Jamil Mukulu, a Christian turned Muslim.

The group has been in eastern DRC since 1995 and is thought to have killed at least 700 civilians. It is accused by the U.N. and Congolese authorities of committing a series of civilian massacres since 2014, but 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 militia has been blamed for a recent string of attacks in the region.

On November 18, the World Health Organsation said efforts to fight a deadly Ebola outbreak in the Beni region have resumed after a brief suspension following clashes that erupted two days earlier just a “few meters” from a local emergency center and the hotels of several response teams.

Monusco peacekeepers repelled the ADF offensive in Beni city’s northern Boikene neighborhood, a ministry statement said.

Attacks have hindered efforts to control the worst Ebola outbreak ever in DR Congo. More than 380 people have been infected and two-thirds of those have died, Reuters reported.

Two raids on November 4 that killed at least seven people in the North Kivu region were also blamed on the ADF. Fifteen people, including children, were kidnapped in those attacks.

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.

With reporting from AFP