Jailed Russian opposition leader Alexey Navalny has been sentenced to 19 years in prison after being found guilty on extremism charges, a court within the high-security prison where he is being held, said on Friday, according to Russian state media TASS.

This latest verdict is a fresh blow to a fierce critic of Russia's President Putin that comes amid a worsening climate triggered by the Ukraine war.

TASS said Navalny was found guilty on charges related to creating, financing, organizing and participating in "an extremist community," as well as for public calls for extremist activities and involving minors in dangerous activities.

He will serve his sentence in a special regime colony. The Russian state media agency described the high-security prisons as containing "convicts with especially dangerous recidivism and sentenced to life imprisonment." Navalny faces greater restrictions on how he can spend free time, communicate and move around the prison.

About Navalny: Navalny has been incarcerated in Russia since his return to the country in January 2021, on charges of violating terms of probation related to a years-old fraud case, which he dismisses as politically motivated.

There have been concerns about his well-being: Navalny lost weight and suffered stomach pain earlier this year, leading to fears among his lawyers that he had again been poisoned.

He had previously been taken from Russia to Germany in August 2020, after he was poisoned with the Soviet-era nerve agent Novichok.

A joint investigation by CNN and the group Bellingcat implicated the Russian Security Service (FSB) in Navalny’s poisoning, piecing together how an elite unit at the agency had followed the opposition leader’s team throughout a trip to Siberia, when he fell ill. Russia denies involvement in Navalny’s poisoning.

Criticism of the war in Ukraine: Although the Russian authorities’ targeting of Navalny pre-dates Moscow’s invasion of Ukraine, the country has cracked down even more dramatically on internal opposition and free speech since launching the war.

An expanded and intentionally vague law on “foreign agents” came into effect late last year, requiring organizations and individuals engaging in political activity and receiving funding from abroad to adhere to draconian rules and restrictions.

It has also restricted social media and Western news access, clamped down on peaceful protests, and criminalized the spread of what it calls "deliberately false" information about Russia's military.

Navalny has nonetheless been a vocal critic of the conflict. On the anniversary of the invasion in February, he called it “an unjust war of aggression against Ukraine under ridiculous pretexts.”