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Colombian President Juan Manuel Santos has condemned as "cowardly" an ambush by left-wing Farc rebels that has killed a senior police officer.

Maj Felix Antonio Jaimes Villamil was killed by an explosive device in north-western Antioquia province.

Police said the explosives were set off as Maj Jaimes arrived to investigate a rebel attack on two buses.

The Colombian authorities say the Farc have stepped up their attacks on civilians and security forces alike.

Police say three people were injured early on Wednesday when members of the Revolutionary Armed Forces of Colombia (Farc) burned down two buses and a lorry on the main road linking Colombia's second city, Medellin, with the Atlantic Coast.

Officials said Maj Jaimes, the commander of Antioquia's highway police, travelled to the area to oversee the evacuation of the victims.

Two police agents who were accompanying Maj Jaimes were also injured when the device went off.

President Santos called the ambush "terrorist and cowardly".

"This shows that the Farc are ever more desperate and weak," the president said.

Colombian security forces have killed a number of Farc leaders over the past year, and arrested many others.

The group's top military commander Jorge Briceno, better known as Mono Jojoy, was killed in an army bombing raid last September.

And the army says it is getting ever closer to Farc leader Alfonso Cano, whose head of security it killed in March.

But the rebels also have been stepping up their attacks on both civilian and military targets.

The Colombian hauliers' union has asked the government to increase security on the country's highways.

They said the attack was the ninth so far this year in Antioquia, with some 40 vehicles burned by the guerrilla nationwide.