* Recent kidnappings could complicate peace talks * Five soldiers killed in combat with FARC in south * Oil sector has been under increasing attacks * Six rebels killed during operations in the north By Helen Murphy and Jack Kimball BOGOTA, Jan 31 (Reuters) - Colombia's FARC rebels on Thursday freed three kidnapped oil contractors, but six guerrillas and five government soldiers were killed across the country as peace talks continued abroad. The kidnappings and other vi