FILE PHOTO: Russian opposition politician Alexei Navalny attends a court hearing in Moscow, Russia February 20, 2021. REUTERS…
FILE - Russian opposition politician Alexey Navalny attends a court hearing in Moscow, Russia, Feb. 20, 2021.

MOSCOW - A doctor for imprisoned Russian opposition leader Alexey Navalny, who is in the third week of a hunger strike, says his health is deteriorating rapidly and the 44-year-old Kremlin critic could be on the verge of death.

Physician Yaroslav Ashikhmin said Saturday that test results he received from Navalny's family showed him with sharply elevated levels of potassium, which can bring on cardiac arrest, and heightened creatinine levels that indicate impaired kidneys.

"Our patient could die at any moment," he said in a Facebook post.

Anastasia Vasilyeva, head of the Navalny-backed Alliance of Doctors union, said on Twitter that "action must be taken immediately."

Navalny is Russian President Vladimir Putin's most visible and adamant opponent.

His personal physicians have not been allowed to see him in prison. He went on a hunger strike to protest the refusal to let them visit when he began experiencing severe back pain and a loss of feeling in his legs. Russia's state penitentiary service has said that Navalny is receiving all the medical help he needs.

Navalny was arrested on January 17 when he returned to Russia from Germany, where had spent five months recovering from Soviet nerve-agent poisoning that he blamed on the Kremlin. Russian officials denied any involvement and even questioned whether Navalny had been poisoned, though it was confirmed by several European laboratories.

He was ordered to serve 2½ years in prison on the ground that his long recovery in Germany violated a suspended sentence he had been given for a fraud conviction. Navalny said that case was politically motivated.