The attack late Thursday in the city of Tame, in northeastern Arauca department near the Venezuelan border, occurred as soldiers were driving across a bridge. The army statement said three wounded soldiers have been hospitalized.
The government of President Juan Manuel Santos and ELN, the National Liberation Army, opened peace talks on February 7 in Quito, Ecuador after nearly four years of secret negotiations.
The ELN, with 1,500 fighters, is the last guerrilla group still active in Colombia.
The government is seeking a "complete peace," after reaching a peace accord last year with the much larger FARC -- the Revolutionary Armed Forces of Colombia.
The Colombian conflict erupted in 1964 when the FARC and ELN took up arms for rural land rights.
The fighting, which over the years drew in various rebel and paramilitary groups and drug gangs as well as state forces, has left at least 260,000 people dead, according to the authorities.