The heaviest fighting took place on Saturday in the southwestern Putumayo state, one of Colombia's main cocaine-producing regions, where 19 soldiers were killed, an army spokesman said on condition of anonymity in line with government policy.

The fighting erupted after the Revolutionary Armed Forces of Colombia, or FARC, attacked an army convoy near Puerto Asis, some 530km southwest of Bogota, the spokesman said.

Separately, government troops backed by warplanes clashed with leftist rebels blocking a road in northeast Colombia, killing at least six soldiers, General Edgar Ceballos, the local army commander, said.