MOSCOW (AP) — A lawyer for imprisoned Russian opposition leader Alexei Navalny, who has complained of serious back and leg pain in custody, said Wednesday that doctors have found him to be suffering from two spinal hernias.

Vadim Kobzev told the Interfax news agency that Navalny also has a spinal protrusion and is beginning to lose feeling in his hands.

Navalny went on a hunger strike last week to protest what he called poor medical care in a Russian prison. On Tuesday, the leader of the Navalny-backed Alliance of Doctors union was detained by police after trying to get into the prison to talk to doctors.

Navalny, 44, is Russian President Vladimir Putin’s fiercest domestic opponent. He was arrested in January upon returning to Moscow 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. Still, labs in Germany and elsewhere in Europe confirmed that Navalny was poisoned with the Soviet-era Novichok nerve agent.

ALEXEI NAVALNY 'SERIOUSLY ILL' IN PRISON, MOVED TO SICK WARD: REPORT

A Russian court ordered Navalny in February to serve 2 1/2 years in prison for violating the terms of his probation, including when he was convalescing in Germany, from a 2014 embezzlement conviction. Navalny has rejected the conviction as fabricated, and the European Сourt of Human Rights found it "arbitrary and manifestly unreasonable."

Navalny said in an Instagram post Wednesday that prison authorities have tried to undermine his hunger strike by allowing aromatic chicken to be cooked in his unit’s kitchen, a departure from regulations, and by placing sweets in his clothes pockets.

"Do you know what turned out to be most important in the first stage of the prison hunger strike? Check your pockets," he wrote..

Navalny’s imprisonment has brought wide criticism from the West. White House spokeswoman Jen Psaki said Wednesday: "We urge Russian authorities to take all necessary actions to ensure his safety and health," adding that "we consider Mr. Navalny’s imprisonment on trumped-up charges to be politically motivated and a gross injustice, and we stand with like-minded allies and partners in calling for his immediate release."

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

FILE PHOTO: Russian President Vladimir Putin gives a thumbs-up as he attends a foundation-laying ceremony for the third reactor of the Akkuyu nuclear plant in Turkey, via a video link in Moscow, Russia March 10, 2021. Sputnik/Alexei Druzhinin/Kremlin via REUTERS ATTENTION EDITORS - THIS IMAGE WAS PROVIDED BY A THIRD PARTY./File Photo

FILE PHOTO: Russian President Vladimir Putin gives a thumbs-up as he attends a foundation-laying ceremony for the third reactor of the Akkuyu nuclear plant in Turkey, via a video link in Moscow, Russia March 10, 2021. Sputnik/Alexei Druzhinin/Kremlin via REUTERS ATTENTION EDITORS - THIS IMAGE WAS PROVIDED BY A THIRD PARTY./File Photo

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Within weeks of being imprisoned, Navalny said he developed severe back and leg pains and was effectively deprived of sleep because a guard checks on him hourly at night. He went on a hunger strike on March 31, demanding access to proper medication and a visit from his doctor.

Russia’s state penitentiary service has said that Navalny is receiving all the medical help he needs.

Another Navalny lawyer, Olga Mikhailova, said a neurologist consulted by Navalny’s organization said the treatment prescribed in the prison was ineffective.

Navalny said Monday that three of the 15 people he is housed with have been diagnosed with tuberculosis, a contagious disease that spreads through the air. He said he had a strong cough and a fever with a temperature of 38.1 degrees Celsius (100.6 F).

On Monday, the state penitentiary service said Navalny had been the prison’s sanitary unit after a checkup found him having "signs of a respiratory illness, including a high fever."

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Mikhailova said Wednesday that Navalny’s fever had lowered, but he is still coughing and is weak from the hunger strike.