Prominent opposition politician Ilya Yashin has been moved from prison camp number three in the Smolensk region to an undisclosed location, his lawyer Tatyana Solomina said on Tuesday.
Independent Russian media have previously reported on the transfer of other political prisoners.
As of Tuesday afternoon, there were six confirmed transfers, including the human rights activist Oleg Orlov from the organisation Memorial and the artist Alexandra Skochilenko.
All of them are critics of Russia's full-scale invasion of Ukraine and received long sentences.
The United States and its allies have criticised the sentences as arbitrary and demanded the release of the prisoners.
Reports indicate that the former heads of the regional staff for Kremlin critic Alexei Navalny, who died in February in Russian prison camp north of the Arctic circle, have also been moved to undisclosed locations.
Currently neither lawyers nor relatives have information on the whereabouts of Liliya Chanycheva and Xenia Fadeeva, both of whom worked for Navalny.
"It seems that we are facing a very big exchange with the Americans (and not only there)," political scientist Tatyana Stanovaya wrote on her Telegram channel.
She did not elaborate on her statement but posted the message amid increasing news about the disappearance of imprisoned Kremlin opponents.
"We all hope that these are good signs," Ivan Pavlov, a prominent human rights activist who fled Russia and is now based in Prague, told Reuters.
"We hope that they (the authorities) have probably taken them all out of their prisons to gather them together in one place in preparation for an exchange."
Pavlov, whom Russian authorities have designated "a foreign agent," said the prisoners were most likely to have been taken to Moscow's Lefortovo Prison.
President Vladimir Putin would then need to formally pardon them before they were put on a plane to a destination in Europe, which Pavlov said could be in Germany.
Putin, who has been criticised for using political prisoners as hostages to free Russians from foreign prisons, has recently repeatedly declared his willingness to make an exchange.
The Kremlin declined to comment on Tuesday on the possibility of a prisoner swap involving Wall Street Journal reporter Evan Gershkovich.
Russia said last month that contacts had taken place with the United States over a possible prisoner exchange involving Gershkovich but that the talks should take place far from the media.
A Russian court found Gershkovich guilty of espionage on July 19 and sentenced him to 16 years in jail.
He denies the charges against him and the Wall Street Journal branded the sentence "a disgraceful sham conviction".
with Reuters
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