At least seven other prisoners held in Russia were reported to have been moved from their jails or prison colonies in the past few days, fuelling speculation of an imminent prisoner exchange.

"I receive requests from journalists from various international agencies asking me to clarify whether I know Paul's whereabouts, as there are rumours of a possible exchange," his lawyer Olga Karlova told the Interfax news agency.

"I have sent a request to the (prison) colony administration, but I'm not getting an answer," she said.

Whelan, 54, was detained on spying charges while visiting Moscow in December 2018, claims he has denied.

He has been jailed in Russia since, receiving a 16-year prison sentence at a trial in 2020. His family and supporters have campaigned intensely for his release, saying his condition has deteriorated during incarceration.

Arrests of US citizens in Russia have increased in recent years, in what Washington sees as a Kremlin attempt to secure the release of Russians convicted abroad.

Other US nationals detained in Russia include reporters Evan Gershkovich and Alsu Kurmasheva, ballerina Ksenia Karelina, another another former marine, Robert Gilman.