Imprisoned Russian opposition leader Alexei Navalny has been relocated from the penal colony where he was serving his sentence, and his current whereabouts are unknown, according to his spokeswoman, Kira Yarmysh.

Yarmysh posted on social media that authorities at the Melekhovo colony confirmed that Navalny was no longer registered there. His aides had anticipated his transfer to a more stringent prison facility after an extension of his prison term.

The disappearance of Navalny, they believe, is connected to recent announcements regarding upcoming elections. The White House expressed deep concern over these reports.

In August, Russian President Vladimir Putin declared his intention to seek re-election in March. Navalny, found guilty of founding and funding an alleged extremist organization (which he denies), was handed an additional 19-year sentence. Previous charges against him, including parole violations, fraud, and contempt of court, are widely regarded as politically motivated.

He was informed that his continued incarceration would take place in a "special regime colony," typically reserved for dangerous criminals, repeat offenders, and those serving life sentences. Navalny's associates revealed on Monday that they had not received any information about him for the past six days.

He had been absent from several recent court appearances, which he has in the past attended by video link. Prison authorities blamed the absences on technical problems at the colony.

Then Ms Yarmysh said lawyers waiting for news outside Melekhovo, 235 km (145 miles) east of Moscow, and another nearby colony, had been told that he was not registered in either place.

His aide Leonid Volkov said this was "0% coincidence and 100% direct manual political control from the Kremlin".

"It's not a secret for Putin who his main opponent is in these 'elections'," he said.

"And he wants to make it so that Navalny's voice is not heard. So each one of us must become the voice of Navalny."

For more than a decade, Mr Navalny sought to expose corruption at the heart of Russian power. His video investigations have received tens of millions of views online.

A charismatic campaigner, he seemed to be the only Russian opposition leader capable of mobilizing people in large numbers across Russia to take part in anti-government protests.

But in 2020, he was poisoned in Siberia by what Western laboratories later confirmed to be a nerve agent.

A later report by the investigative outlet Bellingcat and Russian news site The Insider implicated several agents of Russia's internal security service, the FSB, in the attack.

After recovering from the attack, Mr Navalny returned to Russia in 2021 despite warnings that he could face arrest. He was immediately arrested upon arrival at an airport in Moscow.

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