Russia has described as a "blatant injustice" Thailand's decision to extradite alleged Russian arms dealer Viktor Bout to the US to face charges of conspiring to sell weapons.
Russian Foreign Minister Sergei Lavrov said it was the result of unprecedented American pressure on Thailand.
Mr Bout arrived in New York late on Tuesday after Thailand backed the US request following a long legal battle.
He was arrested in Bangkok in 2008 in a US-led operation.
The former Russian air force officer, 43, has been accused of trying to sell arms to Colombian rebels, and supplying weapons that fuelled conflicts in Africa and the Middle East.
If convicted, he faces a maximum penalty of life in prison.
But he denies being, or ever having been, an arms dealer - and Moscow also insists he is innocent.
The 43-year-old is thought to have knowledge of Russia's military and intelligence operations, and Russian diplomats fear the revelations he might make in open court, correspondents say.
Merchant of Death?
Mr Lavrov reiterated Russia's belief that the US pursuit of Mr Bout was politically motivated, and said Russia would use all legal means to support him.
"Contrary to two rulings by a Thai criminal court which concluded that Viktor Bout's guilt was not proven, he has still - by a decision of the Thai government - been extradited to the United States," Mr Lavrov told Rossiya TV on Tuesday.
"I consider this to be unprecedented political pressure on the judicial process and on the government of Thailand. This whole story is an example of blatant injustice. We, as a state, will continue to render all necessary assistance to Viktor Bout as a Russian citizen."
Mr Bout was flown out of Bangkok on a charter flight hours after Prime Minister Abhisit Vejjajiva said the Thai cabinet had backed an appeal court ruling which cleared the way for his extradition.
Dozens of police officers watched over the operation as Mr Bout was transferred from a maximum-security prison to the airport.
His wife, Alla Bout, who has been a frequently tearful figure at Mr Bout's court hearings over more than two years of detention since he was arrested, appeared outside the prison in an apparent attempt to see her husband before he left - but she was too late.
Mr Bout later arrived at the Stewart Air National Guard Base in Newburgh, north of New York City, AP news agency reported citing officials.
Both Mrs Bout and the Russian embassy expressed surprise at the sudden nature of the extradition.
"Nobody knew about this [the extradition], neither I nor the lawyer," Mrs Bout told Russia's Rossiya Television.
"I came to the prison but Viktor was no longer there.
"This is clearly a decision that has been lobbied for by the US and taken under US pressure because only several days remained till Viktor's term was up and, under the law, he was supposed to be freed because from the legal point of view, from the point of view of legality, we had every ground to win this case."
In remarks quoted by Russian news agency Interfax, Mr Bout's attorney Viktor Burobin alleged the extradition was illegal "because the Thai court never reviewed the second US extradition request".
A Russian embassy official told the BBC that the Russian consul had also been unable to see Mr Bout.
Mr Abhisit has faced a difficult dilemma over Mr Bout's case, says the BBC's Vaudine England in Bangkok - whether to co-operate with long-standing ally the United States or to appease Russia, which has a growing tourist and business presence in Thailand.
The courts went back and forth, promising and then delaying the extradition.
A Thai court ruled in August that the extradition should go ahead within three months.
The US even sent a plane to pick him up - but that move proved to be over-confident, our correspondent says.
The courts delayed the extradition again, saying that other charges of money laundering and fraud, earlier laid by US prosecutors, had to be examined.
In October, the court decided to drop those charges, clearing the way for extradition.
Viktor Bout first came to prominence a decade ago when he was described in a United Nations report as "a well-known supplier of embargoed non-state actors" - the UN's way of describing an arms supplier to rebels.
Dubbed the Merchant of Death by a British politician, he was alleged to have supplied arms to Angola, Liberia and the Democratic Republic of Congo.
But he is also suspected of having used his network of air freight companies to supply weapons, in the early 1990s, to Afghanistan and Bosnia.
A website that describes itself as "The official site of Viktor A Bout" says he is a businessman with an undying love for aviation and an eternal drive to succeed.
The website says he started his career in the army of the former Soviet Union - and it was when the Soviet Union collapsed that he started buying up surplus Antonov and Ilyushin cargo planes.