Two Britons are among a group of eight foreign tourists taken hostage by suspected left-wing guerrillas in Colombia.
FARC are suspected to be responsible
Fighters from the Revolutionary Armed Forces of Colombia (FARC) are thought to have taken the tourists, also including four Israelis, a German and a Spaniard, as they trekked near the Lost City (Ciudad Perdida), popular ancient ruins.
The UK Foreign Office named the men as Matthew Scott, 19, from London, and Mark Henderson, 31, from Lincoln, and said consular staff were on their way to the region where the kidnap took place in the far north of the country.
Local reports said a massive manhunt was on for the kidnappers and their victims with up to 1,500 police and soldiers as well as nine Black Hawk helicopters joining the search.
The attack took place in the mountainous area of the Sierra Nevada, inland from the Caribbean resort of Santa Marta.
The area is not one of those that the Foreign Office strongly advises against all travel to, but British tourists are told that overland travel in any part of Colombia is unwise.
Sierra Nevada is a disputed territory, fought over by Marxist guerrillas, right-wing paramilitaries and drugs traffickers, none of whom welcome foreign visitors, says the BBC's Jeremy McDermott in Bogota.
Most of Colombia's 3,000-odd kidnappings every year - that is one every four hours - are carried out by FARC, who use the ransom to fund their 39-year war on the state.
Discovered in the 1970s, the Lost City is made up of a web of roads, terraces and small stone plazas thought to date from 500 BC. It is only accessible by a five-day trek or hired helicopter.
Alistair Taylor, an oil worker from Aberdeen who spent almost two years as a hostage of left-wing guerrillas, said the Britons would be going through "sheer hell" as their captors tried to evade pursuers.
He told BBC News Online: "They will be marched, they won't get a minute's rest.
"The guerrillas are going to be as scared as them. They need to get them hidden, the army is looking for them. They will be under a lot of pressure, keeping them moving."
But he said the most terrifying moment would come if government forces caught up with the rebels and attempted to free the hostages using force.
Ruth Skipsey of Journey Latin America said few British tour operators would organise hiking excursions anywhere in Colombia.
"The advice is to avoid overland travel in most areas. It is fine to fly into Bogota [the capital] but then you are very restricted."