BOGOTA, Colombia, Nov 7 (Reuters) - The Colombian military has a good idea where Marxist rebels are holding three U.S. Defense Department contractors hostage but has not attempted to rescue them for fear of endangering their lives, the Colombian defense minister said on Friday.

"We have a pretty clear idea of where they are keeping them, but the prime concern of our armed forces has been the responsibility to avoid any operation which could put them in serious risk," Defense Minister Marta Lucia Ramirez told reporters.

Thomas Howes, Marc Gonsalves and Keith Stansell were captured by the Revolutionary Armed Forces of Colombia in February when their light aircraft crashed on a southern Colombian jungle hillside while on a mission spying out drug crops.

The rebels, a 17,000-strong group known by the Spanish initials FARC, say the three civilians are CIA agents and have put them on a list of politicians and soldiers they want to swap for guerrillas held in government jails. They killed two other crew members who survived the crash -- a Colombian army sergeant and a U.S. Vietnam veteran.

The Colombian army, assisted by U.S. experts, launched a massive search operation for the men when they were captured, relatively close to a large military base. But the Colombians' record in such operations has been mixed, and FARC guerrillas killed 10 hostages as troops staged a botched rescue attempt in thick jungle in May.

A video of the three Americans, showing them in good health but surrounded by gun-toting rebels in a secret location, was made by a Colombian journalist and broadcast on U.S. television in October.

The men sent messages to their families and said that they were so heavily guarded a rescue would be impossible.

"This isn't a movie, this is real life. I don't believe a rescue is possible," Stansell said.

Civilian contractors, many flying spy planes and crop dusters, play a crucial role in Colombia's U.S.-funded drive to wipe out the cocaine industry. The FARC, fighting a four-decade-old war, draws much of its funds from drugs.

No U.S. Embassy official was immediately available to comment on Ramirez's comments.