The lawyer of a Russian man accused of laundering more than $4 billion through the digital currency bitcoin urged Moscow on Monday to begin negotiations with the United States to include his client in a potential prisoner swap.

U.S. Secretary of State Antony Blinken said in July that Washington had made Moscow a "substantial offer" for the release of American citizens held in Russia, including U.S. basketball star Brittney Griner and former Marine Paul Whelan.

In a letter seen by Reuters, a lawyer for Alexander Vinnik, a Russian who was extradited last month to the United States to face money laundering charges, called on Foreign Minister Sergei Lavrov to begin swap negotiations with Washington.

"Now the only thing that can save Alexander is for the Russian Federation to enter into negotiations with the American authorities within the framework of the exchange of prisoners between the countries mentioned," the letter from lawyer Frederic Belot said.

Belot said he emailed the letter to the Russian foreign ministry. The ministry declined immediate comment.

Vinnik was arrested in 2017 in Greece at the request of the United States, though Moscow has repeatedly demanded he be returned to Russia.

He was extradited to France from Greece where he was sentenced to 5 years in prison for money laundering before he was sent back to Greece and then on to the United States where he appeared before a San Francisco judge early last month.

His lawyer said that Vinnik was held in solitary confinement in France and due to the stress of isolation psychiatrists "have stated that he has a partial loss of memory," the letter said.

Vinnik's lawyer says his client has repeatedly denied and continues to deny all the charges against him.

The U.S. Department of Justice said Vinnik "allegedly owned, operated, and administrated BTC-e, a significant cybercrime and online money laundering entity that allowed its users to trade in bitcoin with high levels of anonymity and developed a customer base heavily reliant on criminal activity."

The maximum penalty for the U.S. charges against Vinnik is 55 years in prison, the U.S. Department of Justice website says, a figure confirmed by Vinnik's lawyer.

"This term for Alexander is tantamount to life," said Belot.

Russia has repeatedly stated that it was engaged in "quiet diplomacy" with the U.S. regarding a potential prisoner swap.

(Editing by Guy Faulconbridge; Editing by Alistair Bell)