Jailed Kremlin critic Alexei Navalny has been transferred out of his prison colony to an unknown location, his aides say.
Kira Yarmysh, the imprisoned Russian opposition leader’s spokesperson, said the move raised fears for his safety.
Mr Navalny has been outspoken against President Vladimir Putin and was sentenced in March to nine years in prison after he was found guilty of large-scale fraud and contempt of court.
Antony Blinken, the US secretary of state, previously described the charges as dubious.
It was reported last month that he faces “up to 15 more years” after new charges were brought.
Speaking on Twitter on Tuesday afternoon, Ms Yarmysh said there had been rumours that Mr Navalny was to be moved to a high-security penal colony but that it was impossible to know when or if he will arrive there.
She added: “The problem with his transfer to another colony is not only that the high-security colony is much scarier.
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“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.”
Mr Navalny, who was born near Moscow in the then-Soviet Union, has long been Mr Putin’s most vocal critic within Russia.
In June last year, a Russian court banned groups linked to Mr Navalny after declaring them “extremist”.
He has spoken out against the war in Ukraine, calling the invasion “stupid” and “built on lies”.
While serving his jail sentence he called on fellow citizens to stage daily protests.
He was arrested in January 2021 after returning to Russia from Germany where he had spent time recovering from nerve agent poisoning.
The Russian opposition leader had fallen ill on a flight to Moscow and was subsequently found to have been poisoned with novichock during a campaign trip in Siberia.
