Bogota - A Colombian air strike killed 18 FARC guerrillas Thursday, the deadliest attack since President Juan Manuel Santos lifted a suspension of air strikes against the group last month, an official said.
The air strike raises doubts over peace talks that resumed the same day between the Revolutionary Armed Forces of Colombia (FARC) and Santos' government.
The stop-start negotiations are aimed at ending the country's decades-long civil conflict that has killed more than 200,000 people.
"It is the first major blow against FARC since President Santos ordered the resumption of air strikes against the guerrillas on April 15," the Defense Ministry official said about the strike, speaking on condition of anonymity.
"The offensive continues until peace is achieved hopefully as soon as possible!" said a tweet from Santos' account after the attack.
Talks that first began in 2012 have been continually derailed by attacks from both sides, including the capture of a government general.
Late last year, the FARC declared a one-sided ceasefire to boost the chances of the negotiations, but Santos ordered a resumption of military strikes following a rebel attack last month.
Santos has declined to declare a bilateral ceasefire until there is a final peace agreement, though he had suspended air raids after rebel attacks ceased.
The attack Thursday involved both military and police and took place in Guapi in the Cauca region of western Colombia, a FARC stronghold and an area popular with drug cartels.
The army targeted a unit of the Revolutionary Armed Forces of Colombia (FARC) that it blames for an attack in November on Gorgona island that killed one of its lieutenants, the source said.
The region was the scene of a guerilla ambush over a month ago that killed 11 soldiers.
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Santos replaced his defense minister Juan Carlos Pinzon Tuesday, a man some analysts said clashed with the president's goal of hammering out a peace deal with the FARC by 2018.
The current ambassador to the United States, Luis Carlos Villegas, was named as Pinzon's replacement.- Future of talks -
Eleven days of talks in Havana, Cuba are slated to discuss compensation for victims of the conflict, but the air strike raises questions about the future of peace talks and increased violence.
"This is a signal problem... at a time when more progress is expected in de-escalation," said Christian Voelkel, a conflict resolution expert at the International Crisis Group.
But Voelkel said he thought it was unlikely the rebels would suspend their unilateral ceasefire that was declared in December.
"The political costs would be too high and no party has an interest in jeopardizing the process," he said.
The negotiations with the country's largest rebel group have reached deals on political participation, illegal drugs and disarmament among other issues.
The FARC was founded in 1964 and has about 8,000 fighters. Officials said the FARC recently met with the National Liberation Army, a smaller rebel group with about 2,500 fighters, as part of peace talks in Cuba.