Kara-Murza, who holds Russian and British citizenship, was jailed for 25 years in April for treason and spreading "false information" about Russia's war in Ukraine.

The 41-year-old is one of a small number of prominent opposition figures who stayed in Russia and continued to speak out against President Vladimir Putin after the Kremlin sent tens of thousands of troops into Ukraine in February 2022.

Kara-Murza was arrested two months after the war began, accused of spreading false information about the armed forces and declared a "foreign agent". His detention came hours after CNN broadcast an interview with him in which he said Russia was run by "a regime of murderers".

He was later charged with treason over speeches he had made about the war, including one to the Arizona House of Representatives in which he said Putin was bombing Ukrainian homes, hospitals and schools.

Moscow says it does not deliberately target civilians, but thousands have been killed in Ukraine.

During his trial, he compared the wording of the charges against him with that used by Soviet dictator Josef Stalin's secret police in the 1930s, when Kara-Murza's own grandfather was sent to a Gulag prison camp in Russia's far east.

Kara-Murza was a close aide to Russian opposition leader Boris Nemtsov, who was shot dead in central Moscow in 2015.

Twice, in 2015 and 2017, he fell suddenly ill in what he said were poisonings by the Russian security services, on both occasions falling into a coma before eventually recovering. Russian authorities denied involvement in the incidents.

His lawyer has said he is in poor health and suffering from polyneuropathy, a nerve disorder resulting from the poisoning incidents.