WASHINGTON, United States - The United States on Saturday voiced deepening concern over the fate of imprisoned Russian opposition leader Alexei Navalny, who has not been heard from since early December.

"We are deeply concerned about the whereabouts of Alexei Navalny, who has now been missing in Russia's prison system for nearly three weeks," Secretary of State Antony Blinken said on X, the former Twitter.

"We once more call for his immediate release and an end to the continued repression of independent voices in Russia."

On Monday, UN rights expert Mariana Katzarova joined a chorus of voices expressing concern at Navalny's "enforced disappearance."

The dissident's lawyers say they have been unable to see or contact him since December 6.

On December 15, Navalny spokeswoman Kira Yarmysh said a court representative told her that he had been removed from a prison near Moscow to an undisclosed location.

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.

Navalny's team has said his possible transfer to another prison has been purposefully timed as the Kremlin gears up for President Vladimir Putin's expected re-election in March.

Navalny, 47, Putin's main political opponent, was barred from running in elections in 2018 due to an old fraud charge that his allies said was politically motivated.

He received a 19-year prison sentence in 2021 on charges of "extremism" after surviving a poisoning attempt on his life, for which he blamed Putin.