A suspected rebel commando targeted the soldiers on a highway in La Montanita, Caqueta department, killing four, local police said yesterday. There was no report of any additional injuries.
Local media have blamed a unit of the Revolutionary Armed Forces of Colombia (FARC) active in the area.
The FARC, with about 7,000 troops, has been fighting Latin America's longest insurgency, spanning almost five decades.
It has been in peace talks with the government since November 2012.
On Sunday, Colombian voters showed lukewarm support for the talks by giving the country's president a majority in Congress, but also electing his conservative rival, ex-president Alvaro Uribe, to the senate.