Assailants attacked the location where the FARC rebel group had a weapons stash in the southwestern town of Caloto, in Cauca department, according to police, who said a splinter group of the Marxist rebels or the smaller National Liberation Army (ELN) could have been behind the violence.
But Cauca police general Edgar Rodriguez said the ELN was to blame. The ELN is currently the only group fighting the Colombian government, though it is seeking to negotiate its own peace deal.
The United Nations said in a statement that a team of their observers, national police and former FARC rebels were ambushed in Caloto.
The disarmament, mainly in June, by the roughly 7,000 members of Colombia's biggest rebel group under the 2016 peace accord brought a halt to the war.
The conflict left a quarter of a million Colombians dead, about 60,000 unaccounted for and seven million displaced.
The FARC was born in May 1964 from a peasants' revolt. Its ranks were made up mostly of country dwellers who rallied behind the group's Marxist-Leninist ideology, with land reform its focus.
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