A roadside bomb planted by leftist rebels killed 10 soldiers as they patrolled in southwestern Colombia on Thursday, the deadliest attack on security forces this year, authorities said. A similar attack killed nine police officers a day earlier.

The new attack occurred at 12:45 a.m. local time Thursday, said the commander of the army's 3rd Division, Gen. Hernando Perez Molina, who blamed Colombia's largest rebel group, the Revolutionary Armed Forces of Colombia, or FARC.

"We know this is the FARC because it fits their modus operandi, and historically they have operated in this zone," said Perez.

The patrol was searching for criminal gangs and far-right paramilitaries when the device exploded.

The bombing occurred in the province of Valle de Cauca, home to some of Colombia's largest drug traffickers, as well as leftist rebels and far-right death squads. Both the security forces and the rebels have increased their presence in the zone in the past year.

The FARC, which partly funds itself through Colombia's massive cocaine industry, has been fighting for almost five decades against the government and far-right paramilitaries.

On Wednesday, nine police officers were killed when a roadside bomb planted by leftist rebels destroyed their passing truck in central Colombia. Officials also blamed the FARC for that attack.