Russia has criticised a US court for sentencing Russian arms dealer Viktor Bout to 25 years in prison.

It has called the ruling groundless and politically motivated and vowed to press the United States to return him to Russia.

“The American judicial system, carrying out an obvious political order, ignored the arguments of the defence lawyers" in convicting and sentencing Bout, the Russian Foreign Ministry said in a statement.

The ministry "considers the verdict of the American court ... to be groundless and biased," it said.

Bout, the subject of a book titled "Merchant of Death", was caught in an undercover sting in Bangkok in 2008 by US agents posing as Columbian guerrillas seeking weapons and later extradited to New York.

His arrest and trial have been an irritant in ties between Russia and the US, which face new uncertainty as Prime Minister Vladimir Putin prepares to return to the presidency for a six-year term in May.

The Foreign Ministry repeated Russia's allegations that Bout is the victim of a smear campaign and selective justice.

"Long before the sentence was pronounced, Viktor Bout was declared by the [US] authorities to be a 'merchant of death' and practically an international terrorist," it said.

It accused unspecified US media of conducting an "absolutely unacceptable ... smear campaign aimed at influencing the jury and the conduct of the trial as a whole".

Bout was convicted in November of two counts of conspiracy to kill US nationals and officers of the United States and one count each of conspiracy to sell anti-aircraft missiles and providing material support to a terrorist organisation.

The judge said sentencing guidelines called for a life sentence, which prosecutors requested, but said the fact that Bout's conduct was a result of a government sting operation was a mitigating factor despite his long history of arms dealing.

Bout's wife Alla, speaking outside the Manhattan court in footage shown on Russian state TV, called the sentence "a victory".

The Foreign Ministry said Russia would continue to seek Bout's return to Russia, which it has called for since his arrest. "This issue will doubtless remain among our priorities on the Russia-American agenda," it said.