Officials say that at least 18 leftist rebels have been killed in a raid in western Colombia.

A military official says the operation in Cauca province was carried against the 29th Front of the Revolutionary Armed Forces of Colombia on Thursday. The official spoke on condition of anonymity because he was not authorized to comment publicly.

President Juan Manuel Santos is expected to provide details early Friday.

The attack is the biggest blow against the FARC since Santos' government resumed air raids on guerrilla camps in April following a rebel attack that left 10 soldiers dead.

A rebel leader known by his alias as Pastor Alape issued a Twitter post calling Thursday's raid a cold-blooded murder in the dead of night.