Andrei Nesterenko, a ministry spokesman, said U.S. pressure was reported by Thai news agencies, the Novosti news agency reports. Viktor Bout, 42, a former officer in the Russian Army, has been in custody in Thailand since March 2008.
"We consider such acts, which are of an illegal nature, unacceptable and incompatible with a fair trial for Bout," Nesterenko said.
Bout is under indictment in the United States on charges he conspired to sell arms to the Revolutionary Armed Forces of Columbia, a guerilla organization. Thailand said its investigation determined the FARC is not a terrorist group since it controls territory.
In the past, Bout has been accused of shipping weapons to former Liberian President Charles Taylor and Libyan strongman Moammar Gadhafi.