(Adds Colombian official's comment, paragraph 5; details)

By Patrick Markey

BOGOTA, July 3 (Reuters) - Colombian police foiled a plan by leftist rebels to attack U.S. military advisers at a popular vacation resort near the capital Bogota, a government official and the U.S. Embassy said on Tuesday.

Police said they arrested three men in Melgar with explosives and a photograph of a diplomatic vehicle on Saturday, a day after the U.S. Embassy warned its citizens away from the town because of a threat of guerrilla violence.

"There were U.S. military trainers targeted," a U.S. Embassy spokesman said.

Conflict with the guerrillas has ebbed under President Alvaro Uribe and attacks in cities are now relatively rare. The rebels have mostly been driven back to rural regions, where they are still fighting Latin America's oldest insurgency.

A Colombian government official, who asked not to be identified, told Reuters the detainees were suspected members of the Revolutionary Armed Forces of Colombia, or FARC, the country's largest rebel group.

The U.S. advisers were living in Melgar as part of their mission at the nearby Tolemaida army base, the Colombian military's main training center, the embassy official said.

More than 300 U.S. military advisers and trainers are working in Colombia, which has received more than $4 billion in aid from Washington since 2000 to fight drug trafficking and leftist rebels who smuggle cocaine to fund their operations.

U.S. officials are rarely victims of attacks or kidnapping, but the FARC has held three U.S. Defense Department contract workers hostage since 2003, when they were captured after their aircraft crashed on an anti-drug mission.

Melgar, known for its holiday homes, swimming pools and nightclubs, is about two hours drive southwest of Bogota in the Andes mountains. The town is considered safe and is popular with middle-class Colombians.

But the embassy on Friday warned Americans to stay away from Melgar because of information they could be targeted by guerrillas and urged those in the town to keep a low profile and avoid bars and restaurants.

The three detained men were being charged with terrorism and rebellion, police said, adding the suspects had grenades, detonation wiring and photographs of various sites in Melgar.

Rebel violence has surged in recent weeks with a string of attacks in Colombia's main port, Buenaventura, a major narcotics route. Last month, bomb blasts in the city killed two people and wounded at least 23.

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