The Revolutionary Armed Forces of Colombia, or FARC, attacked the soldiers near the town of Vista Hermosa in southern Meta province, an army spokesman said.
The soldiers were providing security for another army team which was manually destroying the raw material of cocaine, the army said, without giving further details.
Both the 17,000-strong FARC and illegal far-right paramilitary militias set up in the 1980s to combat them fund their operations through Colombia's lucrative cocaine trade.
President Alvaro Uribe, popular for his US-backed military campaign against the guerrillas, is expected to win a second term in May.
But analysts say the rebels, who killed eight police officers in another battle earlier this month, are eager to show they still have teeth.
"The FARC is likely to intensify its actions as the election approaches," German Espejo, an analyst at Bogota think tank Seguridad y Democracia, said.
"They want to show that Uribe's security policies are not working," he said.
"The more they discredit him, the stronger their negotiation position will be in the event that there are peace talks during his second term."
Government officials gave no comment on the attack, which was the worst against the military since Mr Uribe took over the presidency in late 2002.
Mr Uribe, whose father was killed in a botched kidnapping by the FARC, is in peace talks with the paramilitaries and preliminary discussions with Colombia's second biggest Marxist guerrilla force, the National Liberation Army, or ELN.
The government and the FARC have yet to agree on terms for talks, although both sides say they would like to negotiate an exchange of hostages held by the rebels for guerrillas held in government jails.
Thousands of people are killed and tens of thousands more displaced every year in Colombia's conflict.