Andrew Smulian, 71, was arrested in 2008 in Thailand along with Bout in an undercover sting by US agents posing as Colombian guerrillas looking to buy weapons.

Bout, the subject of a book titled Merchant of Death, was sentenced to 25 years in prison by Manhattan Federal Court Judge Shira Scheindlin in April after being convicted, but still maintains his innocence.

Upon his arrest, Smulian immediately began cooperating with US authorities, Manhattan federal prosecutors said. He testified against Bout at the weapon dealer's trial.

In sentencing Smulian, the judge said that the only reason he was brought into the case was because US authorities had used him to get to Bout.

"He was a conduit used by the government to get to the man they really wanted to catch," Scheindlin said.

Smulian, who has been incarcerated for 50 months, must serve less than a year in prison in order to complete the five-year sentence, his lawyer said.

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

US authorities have said Bout has been involved in trafficking arms since the 1990s to dictators and conflict zones in Africa, South America and the Middle East.