A US judge on Friday ordered Viktor Bout, a convicted Russian arms trafficker characterized by prison authorities as highly dangerous, to be let out of solitary confinement for the first time in 15 months.

Bout was convicted last November in New York federal court, concluding a marathon operation by US authorities to incarcerate the larger-than-life figure popularly dubbed "the merchant of death," in a case that infuriated Russia.

But Judge Shira Scheindlin, who is due to sentence Bout to a hefty prison term next month, said that keeping him in harsh conditions in the Metropolitan Correctional Center in Manhattan violated his constitutional rights.

Overruling the prison authorities, Judge Shira Scheindlin referred to the US Supreme Court in her ruling, ordering Bout to await sentencing among the regular prison population.

"After fifteen months in solitary confinement with extremely minimal human contact and mobility, Viktor Bout requests that he be transferred to general population," Scheindlin wrote.

"Because I 'cannot simply... abandon my duty to uphold the constitution,'" she said, quoting the Supreme Court, "I must grant Bout's request."

The one-time globe trotting arms king has been in a one-man cell at the MCC's special unit since he was brought to the US soil from Thailand in 2010, following a complex sting operation.

Each day he is allowed out for just one hour a day to exercise in another, almost equally small room, where he also remains by himself, the judge said in her ruling.

"The cell has two small frosted glass windows that allow very little natural light or fresh air," Scheindlin wrote.

Once a bulky man with a rounded face, Bout has become dramatically thinner since his original 2008 arrest in Thailand, which was followed by a long legal battle and eventual extradition to New York. At his trial last year he looked almost gaunt and his wife and lawyer complained that draconian restrictions on his freedoms were putting him under huge stress.

When Bout is sentenced March 12, he faces at least 25 years behind bars for conspiring to sell weapons to Colombia's FARC, a US-designated terrorist organization that prosecutors said Bout believed to be plotting to kill US service members.

Prison authorities have sought to keep Bout under the tightest possible control, saying he is too dangerous to be released among other prisoners.

In her ruling, Scheindlin noted that the MCC warden worried about Bout's "ability to acquire vast resources, which could easily affect an escape or harm a lot of people."

Also, the warden underlined "his alleged leadership... (and) his ability to lead other inmates and control what they can do with the outside."

Prosecutors, meanwhile, stressed that the reason for keeping Bout in solitary was because of his reputed relationship with former Liberian president Charles Taylor, the first African head of state to be prosecuted for war crimes by an international tribunal.

The mustachioed Russian is widely believed to have been the world's biggest black market arms dealer in the post-Cold War period, specializing in arming African warlords and dictators. He says he worked exclusively as a private air transporter -- although sometimes carrying legal shipments of arms.

He lived openly in Moscow, where he reputedly had ties to the Russian intelligence services, until he was lured to Thailand by US agents posing as members of the FARC looking to buy weapons.