Alexei Navalny, the jailed Russian opposition leader, has disappeared from prison and his whereabouts are unknown, his allies said Monday.

Navalny was supposed to appear in court via video link on Monday ahead of a transfer to a "special security" penal colony but never did, spokeswoman Kira Yarmysh wrote on X.

Yarmysh said prison officials blamed electrical problems and that lawyers have been unable to access Navalny for nearly a week.

"It is already the sixth straight day that we don't know where Alexei is and what is happening to him," Yarmysh wrote. 

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In a separate post, she wrote that officials told a lawyer outside the prison that Navalny is no longer among its inmates and the official "refused to say" where he was taken.

Navalny, 47, has been serving a 19-year sentence on extremism charges. He was arrested in January 2021 upon his return to Moscow from Germany, where he recuperated from nerve agent poisoning that he blamed on the Kremlin.

Rejecting all charges against him as politically motivated, Navalny has spent months in isolation in a penal colony in the Vladimir region east of Moscow.

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Navalny’s disappearance comes as campaigning began for Russia’s presidential election in March. Russian President Vladimir Putin, 71, confirmed Friday that he would be running to secure another six-year term in office. It would mark Putin’s fifth term serving as Russia’s leader.

Navalny was due to be transferred to a "special security" penal colony, a facility with the highest security level in the Russian penitentiary system.

Russian prison transfers are notorious for taking a long time, sometimes weeks, during which there is no access to prisoners, and information about their whereabouts is limited or unavailable.

The Associated Press contributed to this report.