Russian opposition politician Alexei Navalny has been removed from the penal colony where he had been imprisoned since the middle of last year and his current whereabouts are not known, his allies have said.
Mr Navalny's aides have been preparing for his expected transfer to a "special regime" colony, the harshest grade in Russia's prison system, after he was sentenced in August to an additional 19 years in prison on top of 11-and-a-half years he was already serving.
The process of moving prisoners by rail across Russia's vast territory can take weeks, with lawyers and family unable to obtain information about their location and well-being until they reach their destination. It was unclear if Mr Navalny was already in transit to a new prison.
Mr Navalny's spokeswoman Kira Yarmysh said staff at the IK-6 facility in Melekhovo, 235km east of Moscow, had told his lawyer waiting outside that the opposition leader was no longer among its inmates.
"We don't know where he is right now. He can be in any colony of special regime and there are about around 30 of them in Russia, all over Russia," she told Reuters.
"We will try to go to every colony there is and look for him," she added.
Mr Navalny's allies said they had been unable to locate him for six days.
The White House said the US was deeply concerned by the reports of Mr Navalny's disappearance.
"We're deeply concerned by these reports that he's now been gone for allegedly a week and neither his representatives or his family know where he is," National Security Council spokesman John Kirby said.
"He should be released immediately, he should never have been jailed in the first place," Mr Kirby told journalists traveling with US President Joe Biden aboard Air Force One on a trip to Philadelphia.
Mr Kirby added: "We're going to work with our embassy in Moscow to see how much more we can find out."
His disappearance comes at the start of the campaign period for a presidential election in which Vladimir Putin confirmed on Friday that he would stand for another six-year term.
Mr Navalny's team is gearing up to start an "anti-Putin" campaign, and Mr Navalny's disappearance will not stop the effort, Ms Yarmysh said.
She said: "Right now he is completely alone and he is literally in the hands of people who once tried to kill him. We don't know what they will do.
"This is why it is so important to talk about him and to try to find him as soon as possible. It is about his life, and his health, and about his security."
Navalny aide Leonid Volkov posted on X that the timing was "0% coincidence and 100% direct manual political control from the Kremlin".
He added: "It is no secret to Putin who his main opponent is in these 'elections'. And he wants to make sure that Navalny’s voice is not heard."
There was no immediate comment from the Kremlin.
Mr Putin and his spokesman make a point of never referring to Mr Navalny by name in an attempt to portray him as politically irrelevant. They say he is treated like any other prisoner.
Navalny aide Lyubov Sobol told Reuters: "We are worried about the state of his health and we're trying to find out exactly where he is now, but that is hard to do."
The process of transferring prisoners by rail across Russia's vast territory can take weeks, with relatives and family unable to obtain information about their whereabouts and well-being until they reach their destination.
Despite his incarceration, Mr Navalny has often been able through his lawyers to post trenchant attacks on the Kremlin via social media, describing his ordeal behind bars and condemning Mr Putin over the war in Ukraine.
But his isolation deepened when three of his lawyers were arrested in October on suspicion of "extremist" activity.
The 47-year-old is by far the best-known figure in Russia's opposition.
For years, he has branded Mr Putin and the ruling elite a gang of "crooks and thieves", lampooning them in slick videos watched millions of times on YouTube.
He earned admiration around the world for voluntarily returning to Russia in 2021 from Germany, where he underwent treatment for what Western laboratory tests showed was an attempt to poison him with a nerve agent in Siberia. He was immediately arrested on arrival.
Mr Navalny says the many charges against him, from fraud and contempt of court to a range of "extremist" activities, were all trumped up to silence his attacks on Mr Putin.
Russian authorities view Mr Navalny and his supporters as extremists with links to Western intelligence agencies intent on trying to destabilise Russia.
Mr Putin has warned the West that any meddling inside Russia will be considered an act of aggression.
