GENEVA: A United Nations rights expert said on Monday the "enforced disappearance" of Russian opposition leader Alexei Navalny was alarming, and demanded that Moscow release him immediately.
Navalny, 47, is President Vladimir Putin's main political opponent.
His lawyers have been prevented from meeting him since December 6, and he did not appear for a scheduled court hearing on Friday.
The lack of updates could mean Navalny is being transferred to a harsher prison, following a court ruling earlier this year.
The process can take weeks in Russia, where prisoners are slowly moved by rail between far-flung facilities.
"I am greatly concerned that the Russian authorities will not disclose Mr Navalny's whereabouts and well-being for such a prolonged period of time, which amounts to enforced disappearance," said Mariana Katzarova, the UN Special Rapporteur on human rights in Russia.
Katzarova said she had raised her concerns with the Russian authorities after Navalny's team were told last week he had been removed from the Vladimir region near Moscow on December 11 and taken to an undisclosed location.
Navalny's supporters have sent requests to over 200 detention centres, his ally Kira Yarmysh said on social media.
The Kremlin has refused to say where Russia's most high-profile prisoner is.
Instead, the main court in Vladimir said on Monday it had sent a request to federal and regional prison services "to establish his whereabouts".
'Baseless' charges
Navalny's team has said his possible transfer to another prison has been purposefully timed as the Kremlin gears up for Putin's expected re-election in March.
Navalny was first jailed in 2021, after surviving an attempt to assassinate him by poisoning.
His sentence was extended earlier this year to 19 years, on extremism charges.
Katzarova said the extremism charges against Navalny were "baseless" and warned that detainees face high risks of serious rights violations during transportation.
The independent expert, who was appointed by the UN Human Rights Council but who does not speak on behalf of the UN, stressed: "The term 'extremism' has no basis in international law."
"When it triggers criminal liability it constitutes a violation of human rights," she said.
She condemned "the unrelenting criminal persecution of Mr Navalny".
Three of Navalny's lawyers were arrested in October.
"Mr Navalny and all those arbitrarily detained should be released immediately and provided remedies and reparations for all the harm suffered," Katzarova said.