* Recent kidnappings could complicate peace talks
* Four soldiers killed in combat with FARC in southwest
* Oil sector has been under increasing attacks
By Helen Murphy and Jack Kimball
BOGOTA, Jan 31 (Reuters) - 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. The FARC 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 Jan. 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.
The FARC, the biggest armed group in Latin America, seized two policemen in the southwest last weekend, the government said, in the first kidnapping of security forces since April, when rebels released all officials under their control.
In the first specific rebel response to the two policemen, chief FARC negotiator Ivan Marquez, one of the group’s seven leaders, said on Thursday that: “Right now we don’t have any official report on that incident, if it was or wasn’t the FARC.”
While the FARC has said it would halt kidnapping to fund its war against the government, it never said it would stop taking members of the armed forces as “prisoners of war.”
The government asked rebels on Wednesday to make it clear they are not wasting time at peace talks in Cuba and genuinely want to end the five-decade conflict.
An escalation of hostilities could affect the progress of the peace discussions. Santos has said he wants to achieve an agreement within a year.
“We’re willing to stay at the table until we find a path that leads us to peace. That’s why we said that we will not get up from the table until the desire of the people in Colombia is fulfilled,” Marquez told journalists in Havana.
The rebels took up arms in 1964 as a Marxist agrarian group fighting against social inequality and the concentration of land among a wealthy elite. But they later turned to drug-trafficking and kidnapping to finance their activities.
Santos is credited with some of the harshest blows against the FARC, first as defense minister and then as president, including killing the group’s leader, Alfonso Cano, in 2011.
Over the years, the FARC has held dozens of politicians, police officers and soldiers in remote jungle hide-outs. Hundreds of FARC members are in Colombian prisons.
Since a 2002 U.S.-backed offensive, security has vastly improved in Latin America’s fourth-largest oil producer, attracting billions of dollars in investment as explorers pushed into ever remoter areas in search of crude.
But the FARC stepped up attacks last year on oil and energy infrastructure. Its bombings of pipelines shot up nearly three fold in the January-October period to 142, but attacks against power transmission towers fell by nearly half in the same period to 35.
In the latest incident, Colombia’s army said that the FARC blew up an transmission tower in a rural zone in the Norte de Santander province on Wednesday.