Imprisoned Russian opposition leader Alexei Navalny said he has started a hunger strike to protest authorities’ failure to provide proper treatment for his back and leg pains.

In a statement posted on Instagram, Navalny complained about prison officials’ refusal to give him the right medicines and to allow his doctor to visit him behind bars.

He also protested the hourly checks a guard makes on him at night, saying they amount to sleep deprivation torture.

Navalny"s lawyers and allies raised the alarm over his deteriorating condition last week, saying they feared for his life. He says he suffers with severe pain.

The prison service responded to Navalny’s complaints with a statement saying the inmate had been provided with, “all the necessary medical aid in accordance with his current medical indications”. In a social media post made on his behalf on Monday, Navalny said he could be put into solitary confinement, in conditions “close to torture”, after being reprimanded by prison officials for a series of minor infractions. His offenses included things such as getting out of bed a few minutes too early.

The 44-year-old Navalny, who is President Vladimir Putin’s most outspoken domestic opponent, was arrested in January upon his return from Germany, where he spent five months recovering from a nerve-agent poisoning that he blames on the Kremlin. Russian authorities have rejected the accusation.