Jailed Kremlin critic Alexei Navalny said on Tuesday he has been charged in a new criminal case and faces up to 15 more years on top of his existing sentence if found guilty.

In posts on social media, Navalny said he had been charged with creating an extremist organisation and inciting hate towards 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 towards 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.

In 2020, Navalny was poisoned with a nerve agent during a campaigning trip in Siberia, according to analysis conducted by multiple European medical institutions. After months of medical treatment in Germany, he was arrested for parole violations when he returned to Russia at the start of 2021.

Following the invasion of Ukraine, the Kremlin has accelerated its decades-long campaign to quash and silence Russia’s domestic opposition. Navalny has spoken out against the war, attacking Putin during a court appearance and calling the invasion “stupid” and “built on lies”.