Leftist rebels have ambushed a committee of election officials in central Colombia, killing 12 security force agents who were providing protection for the group.

The attack early Monday took place in the municipality of Guican, the gateway to Sierra Nevada del Cocuy national park. Six more people remain missing, including two electoral officials, Defense Minister Luis Carlos Villegas said.

The National Liberation Army is Colombia's second-largest rebel group. The Cuban-inspired group has been holding exploratory peace talks with government representatives for more than a year.

President Juan Manuel Santos called regional elections held Sunday the safest in decades with a 60 percent reduction in violence compared with the previous vote in 2011.