Colombia's Farc rebel group has denied kidnapping eight foreign tourists who were snatched from their beds in an armed raid last week.
Thousands of Colombian troops backed by helicopters have been scouring the dense jungle of the Sierra Nevada region for the hostages.
But they have so far found no trace of the missing foreigners - four Israelis, two Britons, a German and a Spaniard - who were kidnapped before dawn on Friday from cabins in the archaeological ruins of Ciudad Perdida, or Lost City.
The Colombian government suspects Farc - the Revolutionary Armed Forces of Colombia - of committing the abduction, the worst group kidnapping of foreigners in Colombia in almost 25 years.
But the rebel group issued a statement saying it was not responsible. "It is false that our guerrilla units are responsible for kidnapping the eight tourists," Farc said in a statement posted on its internet site (www.farcep.org).
"The terrorist operations of the state against the tourists in the Sierra Nevada ... are carried out by Colombian military intelligence, with the goal of showing results to (the president) with a staged rescue," it said.
Previously, Farc has admitted its involvement in high-profile kidnapping cases, such as those involving "political prisoners" it hopes to exchange for rebels in jail.
The remote, northern mountains where the eight tourists were kidnapped are the home to a number of guerrilla armies, including the smaller, Cuban-inspired National Liberation Army, known by its Spanish initials ELN.
Farc and the ELN both regularly take hostages, usually looking for ransom money to finance their campaign.
More than 1,000 people have been abducted so far this year in Colombia, far-and-away the most likely place to be kidnapped on the planet. The guerrilla conflict, already in its fourth decade, claims thousands of lives every year.
The commander of the Colombian army, who is overseeing the search for the tourists, said the terrain could make it very difficult to track them down. General Carlos Ospina told the Associated Press: "This could take a long time."
The search for the hostages is further complicated by the fact that far-right paramilitary death squads - paid to kill rebels and rebel sympathisers - also operate in the zone and are regularly accused of kidnapping.
The tourists - who ignored US and foreign travel warnings when visiting the zone - were trying to catch a glimpse of Colombia's "Lost City," a 2,500-year-old ruin located two days by foot from the Caribbean coast.
Despite the dangers, some backpackers were still looking forward to visiting the Lost City.
Michael Hall, a 24-year-old Londoner who has spent the last five weeks travelling through Mexico and Guatemala, said the presence of so many government soldiers in the search mission would mean the rebels will now keep a low profile.
"It's probably the safest place in the world right now," he said. "If the authorities let us up, I'm ready to go. That's what I came for. They say it is one of the most beautiful places in the world."