Authorities said the massacre, which bore the hallmarks of an ultra-right paramilitary killing, was carried out by unidentified gunmen late on Sunday near the town of Heleconia in Antioquia province.

It came a day after Marxist rebels killed 10 people in three incidents elsewhere in the troubled province.

Maria Gil, right, and daughter mourn over the body of her youngest son.

"The bodies of 13 men were tied up and dumped alongside the highway in groups of two or three," said Diego Coley, a doctor at Heleconia's municipal hospital.

"They were all tied up and had bullet wounds to the head."

There was no immediate indication which group may have been responsible for the massacre.

Colombia's civil conflict has claimed more than 35,000 lives in the last 10 years, with right-wing death squads and leftist guerrillas routinely targeting civilians they suspect are collaborating with their enemy.

Revolutionary Armed Forces of Colombia (FARC) rebels murdered five farm workers and slaughtered more than 170 cows in Antioquia on Saturday, an army spokesman said.