Updated

Russia’s prison agency announced Friday that prominent Vladimir Putin critic and opposition leader Alexei Navalny has died at the age of 47.

The Federal Prison Service said in a statement that Navalny felt unwell after a walk on Friday and lost consciousness, according to The Associated Press. An ambulance arrived to try to rehabilitate him, but he died, the statement added.

Navalny's spokesperson said in a post on X that "we have no confirmation of this yet" and "Alexei's lawyer is currently on his way to Kharp" in northern Russia.

Navalny was being held at the IK-3 penal colony, also known as "Polar Wolf," in Kharp, which is considered one of the country's toughest prisons.

FLASHBACK: RUSSIAN OPPOSITION LEADER NAVALNY'S TEAM FINALLY LOCATES HIM IN REMOTE PRISON COLONY AFTER 20-DAY SEARCH

Alexei Navalny in Moscow, Russia

Russian opposition activist Alexei Navalny takes part in a march in memory of opposition leader Boris Nemtsov in Moscow, Russia on Feb. 29, 2020. (AP/Pavel Golovkin)

Previously, Russian authorities had held him at a facility roughly 145 miles east of Moscow. Navalny’s team lost contact with him after he failed to appear in court via video link for a hearing on Dec. 5, kicking off a desperate search until he resurfaced in Kharp around Christmas.

Kremlin spokesman Dmitry Peskov said Putin was informed of Navalny’s death. 

President Biden told reporters at the White House Friday that "we don't know exactly what happened, but there is no doubt that the death of Navalny was a consequence of something that Putin and his thugs did."

Navalny final court appearance

Russian opposition leader Alexei Navalny appeared in court via video link on Thursday, Feb. 15, a day before his reported death at a penal colony in northern Russia. (Russian Federal Penitentiary Service/SOTAvision/AP)

Vice President Kamala Harris said Friday that reports of Navalny's death are "terrible news, which we are working to confirm."

Secretary of State Antony Blinken said Navalny's "death in a Russian prison and the fixation and fear of one man only underscores the weakness and rot at the heart of the system that Putin has built."

FLASHBACK: RUSSIAN OPPOSITION LEADER ALEXEI NAVALNY SENTENCED TO 19 YEARS IN PRISON

Navalny's wife speaks following reports of his death

Yulia Navalnaya, wife of Russian opposition leader Alexei Navalny, speaks during the Munich Security Conference, in Munich, Germany, on Friday, Feb. 16. She said "If this is true, I want Putin and his entire surrounding, Putin’s friends, his government know – that they will have to pay for what they’ve done with our country, with my family, and my husband. And that day will come very soon." (Kai Pfaffenbach/Pool Photo /AP)

Navalny has previously organized anti-government demonstrations and has run for office to advocate for reforms against what he claims is corruption in Russia. He was the victim of an alleged assassination attempt in 2020, when he suffered poisoning from a suspected Novichok nerve agent. 

He remained in a coma for several weeks while doctors in Germany fought to keep him alive. He accused Putin of being responsible for his poisoning.

Navalny then returned to Russia in 2021, when authorities immediately arrested him and later sentenced him to 19 years in prison on extremism charges. His team has repeatedly raised concerns about his treatment following his return and Navalny has said the charges were politically motivated.

Alexei Navalny with family

Russian opposition leader Alexei Navalny, with his wife Yulia, right, daughter Daria, and son Zakhar pose for the media after voting during a city council election in Moscow, Russia, on Sept. 8, 2019.  (AP/Andrew Lubimov)

The remote region where Navalny was being held is notorious for long and severe winters. Kharp is about 60 miles from Vorkuta, whose coal mines were part of the Soviet gulag prison-camp system.

"This was Vladimir Putin's decision, for sure," Fox News contributor Dan Hoffman told "Fox & Friends." "I think Vladimir Putin sees this killing, murder of Navalny first and foremost to demonstrate for his own people, don't mess with me as the elections approach."

Hoffman said Putin's strategy of oppressing critics as a way of gaining leverage with the U.S. "straight-up mafia extortion."

Rebekah Koffler, a strategic military intelligence analyst and the author of Putin’s Playbook, told Fox News Digital that one possible motive for Putin to be behind Navalny's death is to "stick it to Biden who was threatening Putin" about it.

"He’d want to show Biden that he is powerless to stop Putin. Putin is very vindictive. If you tell him ‘don’t do something or else,’ he will do it to challenge you, unless it’s counter to his interests," she said. 

Reactions from world leaders about Navalny's death are pouring in Friday.

"Obviously he was killed by Putin. Like thousands of others who have been tortured," Ukrainian President Volodymyr Zelenskyy was quoted by the AFP as saying.

United Kingdom Prime Minister Rishi Sunak said in a post on X that "As the fiercest advocate for Russian democracy, Alexei Navalny demonstrated incredible courage throughout his life."

Whenever Putin spoke about Navalny, he made it a point to never mention the activist by name, referring to him as "that person" or similar wording, in an apparent effort to diminish his importance, according to The Associated Press.

Koffler says Putin "certainly has a track record of eliminating his political opponents and vocal critics."

"Among such modern-day, high-profile murders are the killing of Anna Politkovskaya and Boris Nemtsov. A Russian journalist and Putin critic, Politkovskaya reported on the atrocities of Russian forces during the Second Chechen War (1999–2009) and was shot and killed in her apartment elevator on Putin’s birthday in October 2006," she said. "Boris Nemtsov, a former Russian deputy prime minister and opposition leader and Putin’s ardent critic, was shot on a bridge near the Kremlin in February 2015."

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And then last August, Yevgeny Prigozhin, the founder of Russia's Wagner Group who challenged the rule of Putin, was killed in a plane crash outside of Moscow. The White House then appeared to formally acknowledge that the government believes Prigozhin was assassinated by Putin. 

Fox News' Rebekah Koffler, Michael Dorgan, Chris Pandolfo, Nana Sajaia and The Associated Press contributed to this report.