Viktor Bout, a Russian arms dealer caught in an undercover sting by US agents posing as Colombian guerrillas, was sentenced to 25 years in prison on Thursday by a judge in New York, prompting charges of political bias from Russia.

Bout, the subject of a book titled Merchant of Death, asserted his innocence during the sentencing, telling the judge through a Russian interpreter, "I never intended to kill anyone. I never intended to sell arms to anyone. God knows this truth."

"These people know this truth," he said, pointing at federal agents sitting in the front row. "They will live with this truth ... God forgive you. You will answer to him, not to me."

Bout, whom Amnesty International says has been involved in embargo-busting arms deals to human rights abusers in Angola, Liberia, Sierra Leone and the Democratic Republic of Congo, was arrested in Bangkok in 2008 after a US Drug Enforcement Administration sting operation and later extradited to New York to face trial.

He was convicted by a Manhattan federal court jury last fall after a three-week trial that centered on charges he agreed to sell arms to people he thought were Colombian militants intent on attacking American soldiers.

Russia said the "groundless and biased" ruling was politically motivated. The Russian Foreign Ministry said it will continue to seek Bout's return to his country.

"The American judicial system, carrying out an obvious political order, ignored the arguments of the defense lawyers," a ministry statement said.

The sentencing comes at an uncertain time for Russia-US ties, with Vladimir Putin's return to the Kremlin next month and US President Barack Obama facing a re-election battle, but analysts said it was unlikely to significantly sour ties.

"There may be additional declarations (from Russia), but I absolutely do not think this will affect relations," said Maria Lipman, an expert at the Carnegie Moscow Center. "This is not a new case and ... the result of the trial was predictable."

Bout's capture came less than a year after the publication of Merchant of Death: Money, Guns, Planes, and the Man Who Makes War Possible, written by investigative journalists Douglas Farah and Stephen Braun. The 2007 book chronicled Bout's life as an arms dealer and how he evaded capture for years.

He was convicted 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 organization.

His attorneys have said they would appeal.

Defense attorney Albert Dayan argued on Thursday that the government's case was based entirely on Bout's promises, rather than his actions, and therefore had "built-in reasonable doubt".

He said the crimes for which Bout was convicted were intended to target "terrorists, people bent on destroying Americans, people who aren't deterred by life sentences. This is not Viktor Bout".

US District Judge Shira Scheindlin noted that federal sentencing guidelines called for a life sentence, due in part to Bout's conviction on a crime of terrorism, but the fact his conduct was the result of a sting was a mitigating factor, despite his long history of arms dealing.

"But for the approach made in this determined sting operation, it is unclear that Mr Bout would have committed the charged crimes," she said.

US informants posed as arms buyers from the Revolutionary Armed Forces of Colombia, known as FARC, and met with Bout in Thailand to buy an arsenal of military weaponry, which prosecutors said he agreed to provide.

Two DEA informants who posed as FARC leaders testified for the prosecution at Bout's trial. A former Bout business associate, Andrew Smulian, also testified for the government after pleading guilty to participating in the FARC deal.

According to prosecutors, in a meeting at a Bangkok hotel with the supposed FARC representatives, Bout agreed to sell the 100 advanced man-portable surface-to-air missiles or the approximately 5,000 AK-47 assault rifles that were discussed.

Reuters