A statement from the group, known as the United Self Defence Forces of Colombia, said the captives were guilty of an attack on the town of Barrancabermeja in which several people were murdered. They were shot and their bodies burned, the communiqué said.

BBC correspondent Timothy Ross reports that the right-wing group has been held responsible for dozens of mass killings and for forcing thousands of people to flee to refugee camps.

A strike in Barrancabermeja has been planned for Friday in protest against the killings, and troops have been sent there to prevent violence. An oil workers' union spokesperson says all schools, shops and businesses are to close from Friday morning and oil refining and pumping will stop.

A government committee, led by the interior minister and the prosecutor-general, is also going to the region to investigate the murders.

Abducted at gunpoint

The victims were seized on May 16 in a poor slum district of Barrancabermeja, an oil town in the north of the country and one of the most violent places in Colombia.

Heavily armed men went from house to house looking for people on their lists of members of rebel militias. More than 30 young men and women were loaded onto three trucks at gunpoint and taken away.

Seven of the victims were found dead nearby the next day, and for three weeks relatives and human rights organisations have tried to persuade the paramilitary to release the others.

The Archbishop of Bogota says the announcement of their deaths is a grave setback for attempts to negotiate an end to Colombia's civil war.

Human rights groups say the authorities have failed to tackle paramilitary violence, which often involves the members of the army and police

In the case of the latest abduction, the local human rights committee claims the army was an accomplice to the kidnapping, setting up road blocks around the area. Earlier this year, two military intelligence officers were convicted of murdering 43 labour leaders in Barrancabermeja.