Washington/Moscow

In a major development, Washington and Moscow have conducted a prisoner swap, exchanging US basketball star Brittney Griner for convicted Russian arms dealer Viktor Bout, held in an American prison for 12 years.

Griner, the 32-year-old championship-winning player with the WNBA's Phoenix Mercury who for years played for a Russian basketball team in the WNBA's off-seasons, had been held in the country since February after she was arrested on drug possession charges at a Moscow airport.

Meanwhile, Bout, popularly known as the "merchant of death", has reached Moscow, the Russian Foreign Ministry confirmed in a statement on Friday.

US President Joe Biden's administration had proposed a prisoner exchange in July.

Thursday night's elaborate swap involved two private planes bringing Griner and Bout to Abu Dhabi airport from Moscow and Washington, and then flying them home.

Footage on Russian state media showed them crossing on the tarmac with their respective teams.

Addressing the media at the Oval Office, President Biden said that "she's safe. She's on a plane. She's on her way home".

 "After months of being unjustly detained in Russia, held under intolerable circumstances, Brittney will soon be back in the arms of her loved ones, and she should have been there all along.

"This is a day we've worked toward for a long time. We never stopped pushing for her release," he said.

Biden also thanked the United Arab Emirates (UAE) "for helping us facilitate Brittney's return".

"I'm glad to say Brittney's in good spirits... she needs time and space to recover," he added.

Bout's release comes after Biden signed an order for his release, commuting his 25-year jail term, in a direct swap for Griner.

In remarks to the media after landing in Moscow, Bout said that "in the middle of the night they simply woke me up and said 'Get your things together' and that was it."

Bout sold arms to warlords and rogue governments, becoming one of the world's most wanted men.

His secretive career was brought to an end by an elaborate US sting operation in 2008, when he was arrested at a hotel in Bangkok.

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He was extradited two years later and has spent the past 12 years languishing in an American jail for conspiring to support terrorists and kill Americans.