TALLINN, Estonia (AP) — An imprisoned Russian opposition figure has been transferred to a maximum security prison in Siberia and placed in a tiny “punishment cell,” his lawyer said Sunday.

Vladimir Kara-Murza Jr., 42, earlier this year was convicted of treason for publicly denouncing Russia’s war in Ukraine and sentenced to 25 years in prison as part of the Kremlin’s relentless crackdown on critics. On Thursday, he arrived at IK-6 — a maximum security penal colony in the Siberian city of Omsk, his lawyer Vadim Prokhorov said in a Facebook post Sunday.

Prokhorov said the transfer from a detention center in Moscow, where Kara-Murza was being held pending trial and appeals, took less than three weeks. Russian prison transfers, usually done by train, are notorious for taking a long time, sometimes weeks, during which there’s no access to prisoners, and information about their whereabouts is limited.

Kara-Murza, a journalist and an opposition activist, was jailed in April 2022. The charges against him stemmed from a speech he gave weeks prior to the arrest to the Arizona House of Representatives in which he denounced Russia’s invasion of Ukraine.

An associate of opposition leader Boris Nemtsov, who was killed near the Kremlin in 2015, Kara-Murza survived poisonings in 2015 and 2017 that he blamed on the Kremlin. Russian officials have denied responsibility.

Kara-Murza rejected the charges against him and called them punishment for standing up to President Vladimir Putin. He likened the proceedings to the show trials under Soviet dictator Josef Stalin.

Prokhorov said Sunday that upon arrival to the penal colony in Omsk, Kara-Murza was immediately placed in a “punishment cell” — a tiny concrete cell where convicts are held in isolation for violating prison regulations.

Prison authorities have regularly sent imprisoned dissidents to such cells in recent months over alleged minor infractions, a practice that is widely considered designed to put additional pressure on Kremlin critics behind bars.

Prokhorov called the news about Kara-Murza’s extreme confinement “worrying” given his deteriorating health, undermined by the poisonings and solitary confinement he had undergone in pre-trial detention.