By Jeremy McDermott BBC correspondent in Bogota

Colombian authorities say at least 16 unarmed civilians were killed on Thursday night in the northern province of Antioquia.

No group has admitted responsibility for the brutal massacre.

The attackers terrorised villagers in the middle of the night

The zone where the killings took place is a battle front, where three of the country's warring factions are fighting for territorial control.

The atrocity occurred when an armed group arrived in the hamlets around San Carlos in the middle of the night.

They began to drag people from their homes, killing them on their porches.

Among the dead was a man aged over 70, four women and three children.

Two of the murdered women were pregnant.

When troops arrived in the zone, they found bodies strewn around the hamlets.

Two of the victims were still clinging to life and were taken by helicopter to hospital.

Violence imitated

The killings have all the hallmarks of the right-wing paramilitaries, which made the massacre of suspected guerrilla sympathisers one of their principal strategies to cleanse areas of rebel support.

But recently the guerrillas of the largest rebel group, the Revolutionary Armed Forces of Colombia (FARC), have begun imitating this brutal tactic.

Security forces have indicated they think the guerrillas were responsible for this latest atrocity.

Authorities cite the fact that earlier in the week FARC guerrillas opened fire on a bus passing San Carlos, killing four people, one of them a child.

This area in eastern Antioquia has long been a front line in the 39-year civil conflict, with illegal armies of the left and right wings fighting for territorial control.

News of the massacre has come as a blow to the government of President Alvaro Uribe, who was elected on the back of his promise to provide democratic security and protect Colombians.

But his tough stance has simply provoked the Marxist guerrillas to new levels of violence.