Moscow has reacted angrily to the guilty verdict handed down to Russian arms dealer, Viktor Bout, by a court in the United States.
Russia questioned its fairness and said it would try to bring Bout home.
On Wednesday, a New York court found Bout guilty of attempting to sell heavy weapons to a Colombian rebel group.
Prosecutors said Viktor Bout, 44, who has been dubbed "the merchant of death", stood to make millions from supplying weapons to the group.
The defence argued he had only wanted to sell two cargo planes.
The former Soviet military officer was arrested in Bangkok in 2008 after a sting operation in which US informants posed as Colombian rebels.
He remained in custody in Thailand for two years, before being extradited to the US to face trial, where prosecutors alleged that he had tried to sell weapons to Colombia's leftist rebel group, the Revolutionary Armed Forces of Colombia (Farc).
On Thursday, the Russian Foreign Ministry accused the United States of breaking international standards during Bout's arrest and interrogation.
It said the US special services had subjected him to "unjustifiably harsh detention conditions" and "controversial methods of a physical and psychological nature that contravene existing international rights standards".
"All these factors have called into question the very facts on which the prosecution was based, and accordingly, the fairness of the verdict itself," the foreign ministry statement said.
It added that it would "take all measures to protect" Viktor Bout's rights.
A ministry spokesman, Alexander Lukashevich, said: "Our goal is to ensure his return home."
Bout will be sentenced on 8 February 2012 and could face a maximum term of life in prison.
The defence has already said that he will appeal against the verdict.
Bout was convicted of conspiracy to kill US citizens and officials, deliver anti-aircraft missiles and provide aid to a terrorist organisation.
"Viktor Bout was ready to sell a weapons arsenal that would be the envy of some small countries," US Attorney Preet Bharara said.
The conviction relates only to the alleged arms sale in Thailand, but US authorities say Bout has sold weapons to dictators and guerrilla forces in Africa, South America and the Middle East.
It is said that Bout began channelling weapons to war-torn parts of Africa during the 1990s.
The UN has named him an associate of former Liberian President Charles Taylor - who is now awaiting judgement for war crimes.
Media reports in the Middle East claim he was a gun-runner for the Taliban and al-Qaeda - allegations he has flatly denied.
Law enforcement agencies pursued him throughout the last decade.
The Hollywood movie Lord of War, starring Nicholas Cage, was loosely based on Bout's life.