MOSCOW (AP) — Imprisoned Russian opposition leader Alexei Navalny said Wednesday 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.

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.

Navalny’s poisoning and conviction have further strained Russia’s ties with the United States and the European Union, which sank to post-Cold War lows after Moscow’s 2014 annexation of Ukraine’s Crimea, its meddling in the 2016 U.S. presidential election, hacking attacks and other actions.

His arrest fueled a series of protests that drew tens of thousands across Russia. Authorities detained about 11,000 people, many of whom were fined or given jail terms of up to two weeks.

Navalny said the August poisoning made him wonder about the cause of his current ailments. He said he had no choice but to start a hunger strike because his physical condition has worsened, with back pains having spread to his right leg and numbness in his left leg.

“What else could I do?” he wrote. “I have declared a hunger strike demanding that they allow a visit by an invited doctor in compliance with the law. So I'm lying here, hungry, but still with two legs.”

Last month, Navalny was sentenced to 2 1/2 years in prison for violating the terms of his probation during convalescence in Germany. The sentence stems from a 2014 embezzlement conviction that Navalny has rejected as fabricated — and which the European Сourt of Human Rights has ruled to be unlawful.

Navalny was moved this month from a Moscow jail to a penal colony in Pokrov in the Vladimir region, 85 kilometers (53 miles) east of the Russian capital. The facility called IK-2 stands out among Russian penitentiaries for its particularly strict inmate routines, which include standing to attention for hours.

Navalny's Instagram also had a picture of a letter to the prison chief, dated Wednesday, in which he announced the hunger strike.

“Every convict has the right to invite a specialist for a check and consultation,” he wrote. “So I demand to let a doctor see me and declare a hunger strike until it happens.”

In a sarcastic reference to the nerve agent poisoning that he blamed on Russia's top security agency, the FSB, Navalny wrote to the prison chief that “given a recent attempt by the FSB operatives to kill me with chemical weapons, which state-controlled medics cast as a ‘metaboliс problem,’ I'm haunted by vague doubts about the cause of my illness and recovery prospects."

Russia’s prison service said last week that Navalny had undergone medical check-ups and described his condition as “stable and satisfactory.” In a statement that followed his declaration of a hunger strike, it claimed that Navalny is being given “all the necessary medical assistance in accordance with his current health indicators.”

But Navalny has complained that authorities only gave him basic painkiller pills and ointment for his back and legs while refusing to accept medications prescribed earlier by his doctor or to share the diagnosis from his examination.

In a note earlier this month, Navalny described his prison as a “friendly concentration camp.” He said he hadn’t seen “even a hint of violence” there but lived under controls that he compared to those described in George Orwell’s novel “Nineteen Eighty-Four.”

Earlier this week, he said he already had received six reprimands — warnings that could lead to solitary confinement — for offenses such as getting up 10 minutes before the wake-up call and refusing to watch a video lecture that he called “idiotic.”

Navalny, whom prison authorities had earlier marked as a flight risk, said he was subject to particularly close oversight, including a guard waking him up every hour at night and filming him to demonstrate he is in the required place.

“Instead of medical assistance, I'm subjected to sleep deprivation torture, being woken up eight times every night,” he said in Wednesday's statement.

The prison service insisted that Navalny has been treated in strict conformity with the law and the night checks are part of a regular routine that “don't disrupt convicts' rest.”

During a video call with Putin on Tuesday, German Chancellor Angela Merkel and French President Emmanuel Macron emphasized the need for Russia to protect Navalny's health and to respect his rights in compliance with the European Convention on Human Rights, according to Macron’s office.

The Kremlin said in its readout of the call that Putin offered an “objective explanation” in response to questions Merkel and Macron asked about Navalny.

Russian officials have rejected U.S. and EU demands to free Navalny and to stop a police crackdown on his supporters. Moscow also has rebuffed a European Court of Human Rights ruling in favor of his release as “inadmissible” meddling in Russia’s home affairs.

Navalny’s associates have urged Russians to sign up for the next protest to demand his release, promising to set a date for the demonstration when the number of people willing to take part reaches at least 500,000 nationwide.

More than 360,000 have registered since a dedicated website opened on March 23.