Supporters of Navalny, who was jailed in 2021 after surviving a poisoning assassination attempt, have sounded the alarm since early this month, when the Kremlin critic was last seen.

Kira Yarmysh said she was told that Navalny, serving a 19-year sentence, was removed from the Vladimir region near Moscow on December 11, signalling a potential prison transfer.

"It is not known where exactly" he was taken, Yarmysh added.

"I want to remind you that lawyers have not seen Alexei since December 6. Why they were not allowed to meet with him, if Alexei was still in IK-6, we do not know," she said, referring to the penal colony where he was last seen.