At least 18 Farc rebel fighters were killed during an operation by Colombia’s army on Thursday, in one of the most deadly confrontations since the guerrillas began peace talks with the government more than two years ago.

The bombing raid, followed by ground combat, took place in Cauca department on the Pacific coast, the army said, a key drug trafficking region where the Revolutionary Armed Forces of Colombia have a large presence.

The Farc in 2012 began negotiations with the government of President Juan Manuel Santos to end five decades of war. The conflict has killed more than 220,000 people and displaced millions.

“A military offensive produced combat in which 18 rebels died,” an army official told Reuters. The military did not immediately report any casualties.

The rebel deaths come a month after the Farc broke a unilateral ceasefire and killed 11 soldiers, a move that prompted the government to restart bombing raids on guerrilla camps that had been suspended as part of the peace talks.