Posted by 
Lawyers for Alexei Navalny say they have lost contact with the jailed Russian opposition leader, who was believed to be imprisoned in a penal colony about 240 kilometres east of Moscow, and his whereabouts are unknown.
Navalny was sentenced to 19 years in prison in August, after he was found guilty of creating an extremist community, financing extremist activities and numerous other crimes.
He was already serving sentences of 11-and-a-half years in a maximum security facility on fraud and other charges he denies.
READ MORE: Major freeway reopens, at least one fighting for life after 30 vehicles crash west of Melbourne
Supporters of Navalny claim his arrest and incarceration are a politically motivated attempt to stifle his criticism of Russian President Vladimir Putin.
Lawyers have made several attempts to get access to two penal colonies where Navalny, who has suffered serious health issues, was believed to be, spokesperson Kira Yarmysh said on X on Monday. They were informed that the 47-year-old was not at either the IK-6 or IK-7 penal colonies, Yarmysh added.
"On Friday and throughout today, neither IK-6 nor IK-7 responded to them," Yarmysh posted, adding that Navalny had been missing for six days. Navalny was last imprisoned in the IK-6 penal colony east of Moscow.
The White House said it was "deeply concerned" about reports of Navalny's disappearance.
"He should be released immediately. He should never have been jailed in the first place, and we're going to work with our embassy in Moscow to see how much more we can find out," John Kirby, US National Security Council spokesman, told reporters aboard Air Force One.
Navalny was due to appear in court via a video link on Monday, Yarmysh said. Prison authorities told Navalny's team he was unable to appear for the hearing due to an electricity issue in the prison.
READ MORE: Woman and baby found dead in home in Perth's north
Yarmysh said she was concerned by Navalny's recent poor health.
"The fact that we cannot find Alexey is especially alarming because last week he became ill in his cell: he became dizzy and laid down on the floor. The colony staff immediately came running, lowered the bed, laid Alexey down and put [him] on an IV," she said on X.
"What it was, we don't know, but given that he was deprived of food, kept in a punishment cell without ventilation and had his walking time reduced to a minimum, it looked like a hungry faint," Yarmysh said.
Navalny's disappearance comes just days after Putin announced he will run for president again in Russia's elections in March 2024, in a move that could see him retain power until at least 2030.
READ MORE: Cyclone Jasper to bring 'gale-force winds' and heavy rainfall to Far North Queensland
A thorn in Putin's side
Navalny posed one of the most serious threats to Putin's legitimacy during his rule, which has spanned more than two decades. He organised anti-government street protests and used his blog and social media to expose alleged corruption in the Kremlin as well as Russian business.
The dissident was taken from Russia to Germany in 2020, after he was poisoned with Novichok, a Soviet-era nerve agent. Navalny had to be airlifted from the Siberian city of Omsk and arrived comatose at a hospital in Berlin.
A joint investigation by CNN and the group Bellingcat implicated the Russian Security Service (FSB) in Navalny's poisoning. The investigation found that the FSB toxins team of about six to 10 agents trailed Navalny for more than three years.
Navalny later duped one of the spies, Konstantin Kudryavtsev, into revealing how he was poisoned. He posed as a senior official from Russia's National Security Council tasked with carrying out an analysis of the poisoning operation, and phoned Kudryavtsev, who provided a detailed account of how the nerve agents was applied to a pair of Navalny's underpants.
READ MORE: Torrential rain smashes decades-old records in South Australia
Russia denies involvement in Nalvany's poising. Putin said in December 2020 that if the Russian security service had wanted to kill Navalny, they "would have finished" the job.
Navalny was immediately incarcerated upon his return to Russia in January 2021, on charges of violating the terms of his probation related to a fraud case brought against him in 2013, which he also dismissed as politically motivated.
He has campaigned from prison against Russia's invasion of Ukraine and attempted to mobilize public opposition to the war.
"We will conduct an election campaign against the war. And against Putin. Exactly. A long, stubborn, exhausting, but fundamentally important campaign, where we will turn people against the war," Navalny said, according to a statement on his website.
When Navalny was in August sentenced to 19 years in a maximum security penal colony, he said "the number of years does not matter."
"I perfectly understand that, like many political prisoners, I am sitting on a life sentence. Where life is measured by the term of my life or the term of life of this regime," he said in a statement.
