Kremlin critic Vladimir Kara-Murza has been transferred to a prison hospital and his lawyers have been denied access to him for two days, his wife said on Friday, raising concerns for the dissident's fate.

Kara-Murza, 42, is serving a 25-year prison sentence on treason and other charges, one of the harshest punishments Moscow has meted out to opponents of Russian President Vladimir Putin, reports AFP.

Twice poisoned in what he alleges were assassination attempts by the Russian security services, concern has grown for his health since the death of opposition leader Alexei Navalny in an Arctic prison colony in February.

"Vladimir Kara-Murza was transferred to a prison hospital. His lawyers weren't allowed access to him," Evgenia Kara-Murza said in a social media post. She said her husband had been transferred from the IK-6 high-security prison colony in the Siberian region of Omsk where he is serving his sentence to a regional prison hospital.

"His lawyers, who had travelled to IK-6 from Moscow and had been waiting from 8:30 in the morning on Thursday, were informed about this in the afternoon after more than five hours of waiting," she said. After travelling to the hospital, they were first told Kara-Murza was not there, and then after visiting hours closed the facility confirmed to them he had been admitted.

Staff there continued to deny access to his lawyers on Friday, saying he had not been fully processed and was then being seen by doctors, Evgenia Kara-Murza said.