Colombia rebels seize three oil workers, blow up energy tower

* New kidnapping could complicate peace talks

* FARC seized police officers last weekend (Adds details, background)

By Helen Murphy

BOGOTA, Jan 31 (Reuters) - Colombia's FARC rebels kidnapped three oil workers and killed four soldiers in the nation's south, while they blew up an energy tower in the north, military sources said on Thursday, in a sign the group is stepping up pressure during peace talks.

The kidnappings and other violent incidents on Wednesday came days after the Revolutionary Armed Forces of Colombia, or FARC, made clear during ongoing peace negotiations in Cuba that it would continue to capture armed forces, which could muddy the talks.

The oil workers were contracted as engineers by Canada's Gran Tierra Energy, according to an email from the Colombian air force overnight. Company officials in Canada and Bogota were not immediately available to comment.

The air force was hunting for the contractors, the statement said. They were believed to be Colombians.

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.

President Juan Manuel Santos' government and Marxist rebels have been engaged in peace negotiations in Havana since November, trying to reach a settlement to a decades-long war that has killed tens of thousands and defied all past attempts for resolution.

The FARC, the biggest and oldest armed group in Latin America, also seized two police patrolmen in a southwestern province last weekend - the first kidnapping of security forces since April, when it released all government officials under its control.

While the FARC has said it would stop kidnapping to fund its war against the government, it never said it would stop taking members of the armed forces as "prisoners of war."

An escalation of hostilities could affect the progress of the peace talks in Cuba. Santos has said he wants to achieve an agreement within a year.

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 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.

Santos is credited for 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.

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 later turned to drug-trafficking and kidnapping to finance themselves.

Over the years, the FARC has held dozens of politicians, police officers and soldiers in harsh conditions in remote jungle hideouts. Hundreds of FARC members are in Colombian prisons.

Its most high-profile kidnapping, French-Colombian politician Ingrid Betancourt, was rescued in 2008 by the military. (Writing By Helen Murphy and Eduardo Garcia; Editing by Philip Barbara)