
"Obviously, we're disappointed in the court decision," State Department spokesman PJ Crowley said, adding Washington was hopeful an appeal would be successful.
"There is a prospect for the Thai government to appeal this lower court verdict," he said. "But what they decide to do will be governed by Thai law."
Known as the "Merchant of Death", Bout was arrested at a Bangkok hotel in March 2008 in a joint US-Thai sting operation after he allegedly agreed to sell arms to the Revolutionary Armed Forces of Colombia (FARC).
The United States lists FARC as a terrorist organization. Moscow had urged Thailand for Bout's return to Russia. Bout's colourful life was reportedly the inspiration for the 2005 Hollywood film Lord of War, starring Nicholas Cage as an international arms dealer.