The National Liberation Army, Colombia's second-largest leftist rebel group, must release a civilian and a soldier held hostage before the government will agree to begin a peace process, President Juan Manuel Santos said on Monday.
The two sides have been holding preliminary talks for more than two years while the government negotiates a peace deal in Cuba with the Revolutionary Armed Forces of Colombia, or FARC, the country's better known leftist rebel group. The talks with FARC look set to reach a final deal in the coming months.
The National Liberation Army (ELN) has been holding civilian Ramon Jose Cabrales, of eastern Norte de Santander province, for five months. Government soldier Jair de Jesus Villar was captured last week in Antioquia.
"We demand the liberation of Corporal Villar and of citizen Ramon Cabrales," Santos said after meeting with security officials in the city of Arauca. "If they [ELN] want to begin any type of negotiation they must liberate these hostages."
The ELN set off six explosives, targeted at an army brigade, in Arauca on Monday. There were no injuries.
FARC leaders said on Monday that peace in Colombia would be "incomplete" if the ELN, which has about 2,000 fighters, does not participate in negotiations.
"The ELN cannot be left out of the peace process," FARC head negotiator Ivan Marquez said.
Colombia's government warned last week that time was running out to begin peace negotiations with the ELN.
The group has battled a dozen governments since it was founded in 1964 and is considered a terrorist group by the United States and European Union. It has continued kidnapping and attacks on infrastructure even during the exploratory talks.
More than 220,000 people have died in the conflict between the government, leftist rebels and right-wing paramilitaries.