Russian opposition leader Alexei Navalny has been moved to a maximum-security prison, authorities have said.

       Allies of the jailed Kremlin critic had raised the alarm when he was transferred from his prison colony to an unknown location without warning on Tuesday.

       But according to media reports, the chairman of a prison monitoring commission later said that Navalny had been moved to the IK-6 prison in the village of Melekhovo.

       Navalny later confirmed his new location on Telegram on Wednesday, saying he was confined to a “strict regime” and in quarantine.

       "Alexei Navalny was taken away from the penal colony No. 2 [in Pokrov near Moscow]," Navalny's chief of staff, Leonid Volkov had said on Telegram.

       "His lawyer, who came to see him, was held at the gate until 14:00, and then was told that there is no such convict," he added.

       "We do not know where Alexei is now and to which colony he is being taken."

       The same statement was shared on Tuesday by Navalny's press secretary Kira Yarmysh.

       "Of course, neither Alexei's attorneys nor his relatives were informed about his transfer in advance," she wrote on Twitter.

       "There were rumours that he was going to be transferred to the high-security penal colony IK-6 'Melekhovo', but it is impossible to know when (and if) he will actually arrive there."

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       Navalny was arrested on his return to Moscow in January 2021, after he spent several months recovering from a nerve agent poisoning that he blames on the Kremlin. Moscow has always denied the allegations.

       He was then imprisoned for two-and-half years for violating the parole conditions of a 2014 fraud charge. He has been serving his sentence at Penal Colony No. 2 in Pokrov, around 100 kilometres east of Moscow.

       In March, Navalny was given an additional nine-year sentence for fraud and contempt of court -- charges that she says are politically motivated.

       His Anti-Corruption Foundation (FBK) has been outlawed as an "extremist" organisation. and many of his allies have been forced into exile.

       Judges have ordered for Navalny to be transferred to a maximum-security prison in Russia for repeat offenders, where his rights to visits will be reduced.

       Last week, Navalny also lost an appeal over a court decision to label him an "extremist" and a terrorist.

       Sergei Yazhan, chairman of the regional Public Monitoring Commission confirmed on Tuesday that Navalny had been moved to the nearby maximum-security prison, around 250 kilometres east of Moscow.

       Russia’s secrecy about prisoner transfers has come under criticism from human rights advocates.

       Volkov -- who himself left Russia while facing multiple criminal cases -- has previously said that Russian authorities want the opposition leader to remain in prison "until Vladimir Putin or Navalny dies".

       "The problem with his transfer to another colony is not only that the high-security colony is much scarier," Yarmysh said on Twitter.

       "As long as we don't know where Alexei is, he remains one-on-one with the system that has already tried to kill him, so our main task now is to locate him as soon as possible."