BOGOTA, March 26 (Reuters) - Colombia said it seized at least 66 pounds (30 kg) of uranium from the country's biggest left-wing rebel group on Wednesday, the first time radioactive material has been linked to the four-decade-old guerrilla war.

The uranium was found in a rural area long considered a Marxist guerrilla stronghold just south of the capital Bogota.

It is being examined by government experts, the defense ministry said in a statement, although it did not say where the material came from or what it could be used for.

An expert on Colombia's cocaine-fueled conflict said rebels of the Revolutionary Armed Forces of Colombia, or FARC, do not have the facilities needed to make a bomb with uranium.

"This appears to have been part of a black market operation that the guerrillas were trying to use to make money," said Pablo Casas, an analyst at Bogota think-tank Security and Democracy.

"This is new for Colombia and could bring the FARC into the major leagues of black market terrorist transactions," he said.

The government said information about the stash of uranium was found earlier this month in computer files left behind by top guerrilla leader Raul Reyes, who was killed in a Colombian bomb strike against a FARC camp in neighboring in Ecuador.

The March 1 raid sparked a major diplomatic dispute between Colombia's President Alvaro Uribe, a conservative ally of the United States, and the left-wing leaders of Ecuador and Venezuela.

Colombia also claims the files show evidence that Venezuelan President Hugo Chavez has given financial support to the FARC.

The United States, which calls the FARC a terrorist group and has long considered Chavez a destabilizing force in Latin America, said the evidence is "disturbing" .

Chavez openly sympathizes with the FARC but says Colombia's accusations are part of a U.S.-backed plot to smear him. He has also questioned how the computer files could have survived the bombing raid.

The FARC took up arms in the 1960s and is now funded mostly by cocaine smuggling and extortion. The group says it is fighting a Marxist insurgency meant to close the wide gap that separates rich and poor in this Andean country.

"No one believes the FARC wants to blow up Bogota to further the revolution," said a diplomat based in the capital who asked not to be named.

"This seems more like a black market action than military action. It shows again how the FARC is behaving more like an organized crime group than a political group," the diplomat said.

The war kills, maims and displaces thousands of Colombians every year. (Reporting by Hugh Bronstein; Editing by Kieran Murray)