Moscow, Russia - Vladimir Kara-Murza, a prominent opponent of Russian President Vladimir Putin who is serving a 25-year sentence, was transferred from a prison colony in Siberia to a prison hospital.

The transfer was reported by Kara-Murza's lawyers and wife on Friday. The condition of the 42-year-old, who has been in poor health for some time, is unclear.

Lawyers for the British-Russian citizen said they were denied access to their client, who has been held at the Omsk prison camp.

In February, Russia's best-known opposition figure, Alexei Navalny, died in a prison camp in the Russian Arctic region. Other political prisoners in Russia also complain of particularly harsh prison conditions that lead to health problems.

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Kara-Murza's wife, Yevgeniya, said that her husband was suffering from a chronic illness following two poisoning attacks. In a post on X, she described how the lawyers had been trying in vain to see the opposition activist since Thursday.

Kara-Murza was sentenced to 25 years in prison in April 2023 on charges of high treason. He was detained in April 2022 after returning to Russia from abroad amid a growing Kremlin crackdown on civil society.

Citing investigative circles, state media claimed he helped organizations from NATO countries undermine Russia's national security in return for payment.

He was also accused of discrediting the Russian army, which alone can carry a lengthy jail sentence.

Kara-Murza, who is internationally recognized as a political prisoner, has criticized Putin's policies and the Russian war of aggression against Ukraine for years.