Pokrov, Russia - Russian dissident Alexei Navalny, one of the most vocal critics of the Kremlin, has disappeared from the penal camp where he was imprisoned, his lawyer and aides said on Tuesday, adding that they feared for his safety.

"Alexei has disappeared, there is no information about where he is," his spokesperson Kira Yarmysh told the YouTube channel Populjarnaja Politika, Russian for "popular politics."

Navalny had been at the Pokrov prison camp but his lawyer said he had not received any information about his current location.

"He is in danger," Yarmysh said, adding that he could be killed in the brutal prison camp system.

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Navalny narrowly survived an assassination attempt with the chemical agent Novichok in August 2020, for which he blames Russian President Vladimir Putin.

Yarmysh also commented on reports that Navalny might have been transferred to Prison Camp 6 in Melekhovo near the city of Kovrov, saying these had not been confirmed.

"We can't believe it until the lawyer sees him," she said.

The report said Navalny's wife Julia did not have any information either.

Prison Camp 6, known for its harsher conditions, is over 90 miles from the Pokrov penal colony.

Yarmysh said Russia's authorities were doing everything possible to make it difficult for lawyers and family members to contact Navalny.

His nine-year sentence for alleged fraud was confirmed by a court in May, making his transfer to a tougher camp legally binding. In Russian detention centers for felons, inmates are rarely permitted to meet relatives, receive parcels and letters, or have access to fresh air.

At the end of May, Navalny said he had received a new indictment by the Russian judiciary for extremism, which could potentially mean another 15-year sentence.

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Navalny made powerful enemies with revelations about corruption and abuse of power in the Russian state. His anti-corruption foundation was classified as extremist in Russia. So far, every indictment against him has ended in a guilty verdict.

US Secretary of State Antony Blinken tweeted that he was "deeply troubled" by the news and added: "Russian authorities should immediately account for his whereabouts and end their campaign of harassment and intimidation against his supporters."

Authoritarianism has been supercharged in Russia since the start of the Ukraine war, with tens of thousands of arrests and threats of long prison sentences for opposition to the invasion.