A new criminal case has been launched against Alexei Navalny, a critic of Russian President Vladimir Putin, the jailed critic said on Tuesday, adding that he faced up to 15 years in prison if found guilty.

In a post on Instagram, Navalny said he had been charged with creating an extremist organization and inciting hate toward the authorities. The charge comes on top of a nine-year sentence he received in March for fraud and contempt of court, before which he was already serving a 2 1/2 year sentence.

"Not even eight days have passed since my nine-year high-security sentence came into force, and today the investigator showed up again and formally charged me with a new case," Navalny said on Twitter.

"It turns out that I created an extremist group in order to incite hatred toward officials and oligarchs. And when they put me in jail, I dared to be disgruntled about it and called for rallies. For that, they're supposed to add up to 15 more years to my sentence," he said.

There was no immediate confirmation of the new charges.

Human rights organizations and Western politicians have condemned the various criminal cases launched against Navalny, who is President Vladimir Putin's most high-profile opponent, in recent years.

Navalny – Putin's most vocal domestic opponent – has been behind bars since returning to Moscow in January 2021 from Germany, where he was recovering from a poisoning attack with a nerve agent that he blames on the Kremlin.