GOMA, Democratic Republic of Congo (Reuters) - Militants attacked two military bases in northeastern Democratic Republic of Congo on Monday, killing two U.N. peacekeepers and injuring several others, the U.N. mission and the army said.

The mission, known as MONUSCO, said it had deployed attack helicopters in response to the raid, and the army said it was battling militants along the main road leading from North Kivu province’s Beni territory to the Uganda border.

The fighting, which began this weekend, is a fresh outbreak of violence for a region plagued by ethnic tensions and massacres that killed more than 800 people between 2014-2016.

Gunmen believed to belong to the Allied Democratic Forces (ADF), a Ugandan Islamist group active near the border, attacked a U.N. peacekeeping base in the village of Mamundioma, over 30 km (20 miles) northeast of Beni, MONUSCO said in a statement.

“This region has seen more than enough violence already,” mission chief Maman Sidikou said. “I reiterate to all these armed groups who continue to threaten and kill civilians, MONUSCO will respond, and it will respond strongly.”

Army spokesman Mak Hazukay said the attacks were carried out simultaneously and hit a U.N. base housing Tanzanian peacekeepers who formed part of an intervention brigade which has a mandate to conduct offensive operations against militants.

Hazukay said later that U.N. helicopters were shelling rebel positions inside Virunga National Park as Congolese troops fought to recover a stretch of road from the rebels.

ADF is believed by authorities to have ambushed and killed many civilians nearby this weekend after a lull in attacks against civilians near Beni this year.

Omar Kavota, who leads an organization that monitors violence in North Kivu, said ADF fighters killed at least three civilians on Saturday by slitting their throats and took another 19 hostages. MONUSCO said that according to initial reports, up to 20 civilians were killed.

Congolese authorities have blamed the ADF for nearly all the massacres between 2014-2016. However, independent and U.N. experts say several armed groups as well as national army commanders have been involved.