Farc have been fighting the government for more than 40 years

A spokesman said the Farc attack came as the elite forces provided security for workers manually destroying coca plants, used to make cocaine.

This is thought to be the deadliest attack the security forces have suffered under President Alvaro Uribe.

A BBC correspondent in Colombia says Mr Uribe will demand a swift response from the already overstretched army.

The counter-guerrilla troops were ambushed by fighters of the Revolutionary Armed Forces of Colombia in one of their southern jungle strongholds near the town of Vista Hermosa, in Meta province, an army spokesman said.

The BBC's Jeremy McDermott says the soldiers from a mobile brigade were ambushed in what sources indicate was a slick and devastating attack.

QUICK GUIDE The Colombian conflict

Mr Uribe won elections nearly four years ago, promising to crack down on the rebels.

The illegal drugs trade is a key source of funding for armed groups, who are estimated to make hundreds of millions of dollars a year from it.

The Farc and the smaller National Liberation Army (ELN) have been involved in a 40-year conflict with state forces and right-wing paramilitary groups.

The Farc has so far refused to negotiate with Mr Uribe, although he has engaged other groups in peace moves.