Allies of jailed Russian opposition leader Alexei Navalny said Monday they have been unable to locate him for six days and that he has likely been transferred.

Navalny's spokeswoman, Kira Yarmysh, said staff at the penal colony where he has been held since the middle of last year told his lawyer that the opposition leader was no longer among the inmates. Prison officials "refuse to say where they transferred him," she said in posts on X, formerly known as Twitter.

Navalny has been behind bars since January 2021. He is serving a 19-year sentence on extremism charges at a maximum-security prison, Penal Colony No. 6, in the town of Melekhovo in the Vladimir region, about 230 kilometers (140 miles) east of Moscow.

Where can Navalny possibly be?

The opposition leader's allies believe he was taken out of the Vladimir region. "He can be anywhere," said Leonid Volkov, another Navalny ally.

Navalny was to be transferred to a "special security" penal colony, the highest security facility in the Russian penal system. Navalny could be transferred to one of several such penal colonies across Russia.

In Russia's vast prison system, it often takes weeks or even months to transfer prisoners between distant facilities by rail. During the transfer, there's no access to the prisoners, and information about their whereabouts is limited or unavailable.

Navalny's contact with the outside world has been severely restricted in recent months, and he has been in and out of isolation cells this year.

US 'deeply concerned' by reports on Navalny

The United States is concerned after allies of jailed Russian opposition leader Alexei Navalny said they had received no news of him or his whereabouts recently, the White House said. 

"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."

A fierce opponent of President Vladimir Putin, Navalny campaigned against corruption and organized large anti-Kremlin protests. He was arrested on his return to Moscow from Germany, where he was recovering from nerve-agent poisoning that he blamed on the Kremlin.

Navalny has since been sentenced to three jail terms and spent months in isolation in the penal colony for alleged minor offenses. He has denied all charges against him as politically motivated.

dh/wd (AP, AFP , Reuters)