A Russian opposition figure, jailed for publicly denouncing his country's invasion of Ukraine, has been transferred to a maximum security prison in Siberia, his lawyer said Sunday.

Vladimir Kara-Murza Jr. was sentenced to 25 years in prison for treason earlier this year as part of the Kremlin's relentless crackdown on dissent.

He was found guilty of spreading "false information" about the Russian army and of having links to an "undesirable organization" in a closed-door trial.

On Thursday, he arrived at IK-6, a maximum security penal colony in the Siberian city of Omsk, and was placed in a tiny "punishment cell," his lawyer Vadim Prokhorov said in a Facebook post.

Omsk is some 2,700 kilometers (1,670 miles) east of Moscow.

Siberia move puts Kara-Murza's health at risk

Prokhorov said placing the 42-year-old in such cells puts his fragile health at risk as he suffers from a nerve condition called polyneuropathy due to two poisoning attempts.

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

The lawyer 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 is a dual Russian-British citizen, a journalist and an opposition activist.

Kara-Murza survived poisonings in 2015 and 2017 that he blamed on the Kremlin, which Russian officials have denied.

He was an associate of opposition leader Boris Nemtsov, who was killed near the Kremlin in 2015.

US speech on Ukraine war led to arrest

The charges against him stemmed from a speech he gave weeks prior to his arrest to the Arizona House of Representatives in which he denounced Russia's invasion of Ukraine.

Russian President Vladimir Putin has unleashed an unprecedented crackdown on dissent since Moscow's troops rolled into Ukraine in February last year.

Kara-Murza rejected the charges against him and called them punishment for standing up to Putin.

He likened the proceedings to the show trials under Soviet dictator Josef Stalin.

In the month following the announcement of Kara-Murza's unprecedentedly long sentence, several international leaders and supporters demanded that Russia allow his release.

mm/nm (AFP, AP)