Two army brigades, backed up by helicopters and aircraft, launched Operation Scorpion at the Teofilo Forero rebel unit last week, an army spokesmen said on Monday.
The rebel group has been responsible for some of the major attacks by the Revolutionary Armed Forces of Colombia (FARC), he said.
The military has engaged in 26 fire-fights with the FARC rebels in the offensive, which occurred in the southern province of Caqueta, in the heart of what once was a demilitarised zone set aside for peace talks with the guerrillas.
No soldier was killed or injured, the army said.
The announcement coincides with news that eight foreigners have been kidnapped by FARC guerrillas, prompting a massive manhunt in a mountainous area north of the capital Bogota.
Cocaine and kidnapping
The Teofilo Forero used to guard rebel peace negotiators. But it was also partly responsible for the collapse of talks in February 2002, when it hijacked a commercial plane and kidnapped a national senator.
President Alvaro Uribe has boosted defence spending and ordered the military to do more to fight the FARC - a 17,000-strong force funded by cocaine and kidnapping, which is fighting a 39-year-old guerrilla war.
But critics say the military has failed to capture or kill any senior FARC commanders and even Defence Minister Marta Lucia Ramirez has complained the armed forces have not dealt major blows to the rebels.
About 200,000 people have been killed in the civil conflict in the South American country since 1964.