Russian opposition politician Vladimir Kara-Murza, jailed for 25 years on treason charges and for denouncing Moscow's Ukraine offensive, has arrived to serve his sentence at a maximum-security Siberian prison, his lawyer said yesterday.

Kara-Murza -- a dual Russian-British citizen -- was handed the unprecedentedly long sentence in April, with international leaders and supporters calling for his release.

"Vladimir Kara-Murza has been brought to the Omsk maximum security penal colony IK-6 to serve his punishment," his lawyer Vadim Prokhorov said on Facebook.

"He was straight away placed in an isolation cell."

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

Russia's penal system often takes weeks to bring prisoners to the country's far-flung jails, with the whereabouts of a prisoner in transit often unknown and stopping in various jails along the way.

"The whole journey from Moscow to Omsk in the 21st century took no less than three weeks," Prokhorov said, saying he was kept for "several days" in an isolation cell in the central city of Samara.