Clashes between Colombia's Farc rebels and the government have left at least nine people dead, as the latest round of peace talks begins in Cuba.
Five rebels are said to have died in a government offensive in the north.
Four government soldiers were reportedly killed and another two wounded in southern Colombia.
Colombian chief negotiator, Humberto de la Calle, said the outbreak of violence would not force the government into agreeing a bilateral ceasefire.
The five rebels were killed as troops moved into the town of Nudo Paramillo, in the northern region of Antioquia, officials said.
Other clashes reportedly happened early on Thursday as security forces tried to stop the rebels from entering the town of Policarpa, in southern Colombia. During the fighting, the four soldiers were said to have died.
Police in Colombia have blamed the Farc for the disappearance of three oil workers on Wednesday, but there has been no confirmation from the rebels.
The contractors, working for Canada's Gran Tierra Energy, were taken hostage in the southern Cauca region, Reuters news agency reports.
The men were picked up unharmed by the armed forces on Thursday, after being left in a forested area.
The military said the hostages were abandoned after they stepped up their operations against the Farc in the area.
Last Friday, two policemen disappeared but Farc chief negotiator Ivan Marquez said he had no knowledge of their whereabouts.
Despite the violence and reported abductions, the fourth round of negotiations resumed in Havana on Thursday.
The left-wing rebel group defended what it said was the group's "right" to capture "prisoners of war".
But Colombia's Humberto de la Calle described the Farc's statement about "prisoners of war" as word-play.
The rebels declared a unilateral two-month ceasefire at the start of the talks in November to end five decades of conflict, but it was suspended on 20 January.
Colombia's stance has been to accept a bilateral truce only when a final peace agreement has been signed.
After the last round of negotiations, a Farc spokesman said the negotiations were advancing at an accelerated pace – "in the rhythm of mambo", as he put it.
But Mr de la Calle has warned that Farc abductions could harm the talks.
The government says the conflict, which has lasted nearly five decades, has cost the lives of more than 500,000 people.
Among the land reform issues proposed by the Farc is a demand that 25 million hectares (62 million acres) of land - more than 20% of the country - be handed over to the country's poor.