At least six Colombian soldiers have been killed and five injured in an ambush in north-western Antioquia province, the army says.
The soldiers where on patrol in a rural area when their vehicle was blown up by a roadside bomb.
Officials said they suspected the Gulf Clan criminal gang to be behind the attack.
The leader of the Gulf Clan, known as Otoniel, was arrested in October after a 10-year manhunt.
A hunt is underway for the attackers. Security forces are also searching for a soldier who is unaccounted for following the explosion.
Colombian radio station Blu posted photos of the lorry the soldiers had been in.
Last year, a police mayor and his driver were killed by a roadside bomb in the same area.
Colombian security forces have labelled the Gulf Clan as the country’s most powerful criminal organisation, while authorities in the US have described it as “heavily armed [and] extremely violent”.
The gang, which operates in many provinces and has extensive international connections, is engaged in drug and people smuggling, illegal gold mining and extortion.
It is believed to have about 1,800 armed members, who are mainly recruited from far-right paramilitary groups. Members have been arrested in Argentina, Brazil, Honduras, Peru and Spain.
The gang controls many of the routes used to smuggle drugs from Colombia to the US, and as far away as Russia.
Attacks on the security forces at the hands of the Gulf Clan have increased since Otoniel, whose real name is Dairo Antonio Úsuga, was captured.
Otoniel is wanted on drug trafficking charges in the United States and is expected to be extradited soon.