TALLINN, Estonia (AP) — Lawyers for prominent Russian opposition figure Vladimir Kara-Murza Jr., who is serving a 25-year sentence for treason, said Tuesday that he has been transferred to another prison in Siberia and placed in solitary confinement again, for at least four months, over an alleged minor infraction.

The move comes amid unrelenting pressure on Russian dissidents at home and abroad that has intensified significantly since President Vladimir Putin sent troops into Ukraine almost two years ago.

Kara-Murza, 42, was held in a prison in the Omsk region, but his supporters said on Monday he apparently was no longer there.

Kara-Murza lawyer Maria Eismont told Russia’s independent Novaya Gazeta newspaper on Tuesday that she had received a letter from him in which he said he was transferred to another penal colony in the city of Omsk and placed in a restricted housing unit for at least four months. In the letter, a copy of which his other lawyer Vadim Prokhorov posted on Facebook, Kara-Murza said that prison officials on Friday accused him of disobeying a command he said wasn’t even given to him.

“So now I’m in the IK-7 (penal colony), also in Omsk,” the politician said in the letter. “It is a special regime colony, there is a special restricted housing unit facility for ‘repeat violators’ like me. I’m in solitary confinement, of course,” he wrote, adding that he was “fine,” had enough food and it was warm in the facility.

Kara-Murza, who twice survived poisonings that he blamed on Russian authorities, has rejected the charges against him as punishment for standing up to President Vladimir Putin and likened the proceedings to the show trials under Soviet dictator Josef Stalin.

According to his wife, Evgenia Kara-Murza, he spent the past four months in solitary confinement, a practice that has become common for Kremlin critics behind bars and has been widely viewed considered designed to put additional pressure on them.

Kara-Murza was arrested in 2022 and later sentenced to 25 years on charges stemming from a speech that year to the Arizona House of Representatives in which he denounced Russia’s full-scale invasion of Ukraine.

Moves to neutralize opposition and stifle criticism intensified after the start of the war in Ukraine, including passage of a law criminalizing reports seen as defaming the Russian military.