Members of the German Bundestag have described the treatment of Russian opposition leader Aleksei Navalny as “targeted torture” and demanded the European Committee for the Prevention of Torture review the conditions of his detention.

The letter, posted on Facebook on April 10, called Navalny’s treatment “incompatible” with the European Convention for the Prevention of Torture, saying Russia is a party to the convention as a member of the Council of Europe.

The letter is signed by Manuel Sarrazin, with Germany’s Green Party, and a bipartisan group of 11 other members of the Bundestag, the lower house in Germany’s parliament. It was made available on Sarrazin’s Facebook page in German and Russian.

“Notwithstanding the arbitrariness and illegality of the judgments pronounced against you, we demand a review of your conditions of detention by the European Committee for the Prevention of Torture,” the politicians said.

The lawmakers said they believe the legal process against him was not carried out under the standards of rule of law and consider the judgment politically motivated and arbitrary with the goal of silencing him.

They said they were following reports about his imprisonment and health condition with great concern and expressed their “full solidarity” with him.

Navalny was imprisoned after returning to Russia in January from his recuperation in Germany after his exposure to a nerve agent last August in Siberia. He has accused Russian President Vladimir Putin of ordering security agents to assassinate him, something the Kremlin denies.

Navalny was treated in Germany after the poisoning, and Sarrazin said that he had the impression that Navalny’s treatment was meant to reverse his partial recovery.

Navalny has complained of back pain and numbness in his hands and legs and accused the authorities of withholding adequate medical treatment.

Navalny declared a hunger strike last week, raising even more concerns about his overall health.

Attorneys for Navalny, 44, said after visiting him on April 8 that he was suffering from two herniated disks in his back and a third bulging disk, and said he is losing about 1 kilogram a day.

With reporting by dpa and Der Tagesspiegel