Colombia's FARC rebels on Thursday freed three oil contractors kidnapped a day earlier, military sources said, though the guerrillas killed four soldiers in the south as they step up pressure during peace talks.
The kidnappings and other violent incidents came days after the Revolutionary Armed Forces of Colombia, or FARC, made clear during peace negotiations in Cuba that it would continue to capture armed forces, possibly hampering the talks.
The FARC released the workers, who were contracted as engineers by Canada's Gran Tierra Energy and were laboring in southern Colombia when they were seized on Wednesday, according to military sources.
There have been several kidnappings of civilians in recent months that are suspected to have been at the hands of the FARC, but the group has never claimed responsibility.
Last year, Gran Tierra quarterly profit fell nearly 59 percent in the second quarter as oil production was hit by pipeline disruptions. FARC members regularly attack oil lines.
President Juan Manuel Santos' government and Marxist rebels have been engaged in peace talks in Havana since November, trying to reach an end to a decades-long war that has killed tens of thousands and defied all past attempts for resolution.
In the southwestern Narino department, a key drug route to the Pacific Ocean, FARC rebels killed four soldiers on Wednesday in combat in the municipality of Policarpa.
Narino department is representative in many ways of the major security challenges facing Colombia - rebel groups and drug gangs sometimes fight or collude to move shipments of cocaine where the government's presence is weak.
At the start of talks in November, the FARC declared a two-month unilateral ceasefire, which ended on January 20 with the rebels attacking oil and mining facilities, including two pipelines and a coal-carrying rail line.
The government refused to join the ceasefire, calling it a sham by the FARC to gain international attention. The army kept attacking the group and carried out several aerial raids that killed at least 34 rebels.