Colombia's main leftist rebel group offered on Monday to swap three kidnapped American defense contractors for two guerrilla leaders jailed in the United States, but the U.S. government immediately rejected the proposal. It was the first time the rebels have expressed a willingness to deal directly with U.S. authorities, but the United States said it won't negotiate with the Revolutionary Armed Forces of Colombia, which the U.S. has blacklisted as a terrorist organization. Tom Howes, Marc Gonsalves and Keith Stansell were captured on Feb. 13, 2003, after their small plane crashed in a rebel stronghold in southern Colombia while on an anti-drug mission. The rebels allegedly killed a fourth American and a Colombian soldier who also were on the plane. "The FARC has informed the U.S. government and the State Department that it's willing to open discussions," Raul Reyes, a spokesman for the group, known as the FARC, told Noticias Uno television. A U.S. official, speaking on condition of anonymity in accordance with policy at the U.S. Embassy in Bogota, said the United States would not negotiate because "such concessions will only encourage future hostage-takings."