Fourteen soldiers were killed in the attack on 24 August.

Four of the six rebels detained were only between 14 and 16 years of age, the army said.

Local army commander Gen Ricardo Gomez Nieto said their arrest was further evidence the ELN was forcibly recruiting minors to its ranks.

"They are torn from their families and turned into instruments of war," the general said.

The security forces say the ambush, which was carefully planned, was a joint operation between Colombia's two largest rebel groups, the Revolutionary Armed Forces of Colombia (Farc) and the ELN.

The joint attack comes three years after the two groups signed a non-aggression pact in the area, in northeastern Colombia near the Venezuelan border.

For the previous five years, the two guerrilla groups had been bitter rivals and hundreds of fighters on both sides had died in their confrontations.

The Farc have been involved in peace negotiations with the government since October last year, but the government has refused to enter into a ceasefire while the talks are under way, arguing that the guerrillas would only use it to rearm and regroup.

Last week, President Juan Manuel Santos said he was ready to open talks with the ELN. But it is not yet clear when the talks with the Marxist group could begin.

A recent study by Colombia's National Centre for Historical Memory suggested 220,000 people have died as a result of the five decades of armed conflict in the country.