The countrys last active rebel group, the ELN, said meanwhile the government had begun releasing certain detainees to clear the way for peace talks with that force, too.

The Revolutionary Armed Forces of Colombia (FARC), the countrys biggest rebel group, is set to disarm in coming months under a peace deal with the government.

"The last march of the FARC has begun," with some 5,000 troops moving in various groups to gather in UN-monitored zones, a government statement said.

Separately, the leftist National Liberation Army (ELN) said the government on Saturday released two of its imprisoned members to serve as peace negotiators - one of the conditions to start talks.

Their release was confirmed on Twitter by ELN negotiators gathered in the Ecuadoran capital Quito, where talks are scheduled to start on Feb 7.

In return, the ELN is to release a former lawmaker it is holding hostage, Odin Sanchez, on Thursday. The government is due simultaneously to release two other imprisoned ELN members and grant them a pardon.

After last years contested peace accord with the Marxist FARC, President Juan Manuel Santos says he wants a deal with the ELN to seal a "complete peace" in Colombia.

The Colombian conflict grew out of a crushed uprising over land rights in the 1960s. It has killed more than 260,000 people and left 60,000 missing, according to authorities.

The conflict has drawn in not only the Colombian armed forces, the FARC and the ELN, but also drug gangs, right-wing paramilitaries and other leftist rebel groups that have since disbanded.