MOSCOW — The Russian authorities moved the imprisoned opposition leader Aleksei A. Navalny to a hospital on Monday for what was described as treatment with vitamins.
The Russian penitentiary system released a statement saying that a commission of government doctors had decided on the move for Mr. Navalny, who is now nearly three weeks into a hunger strike. Mr. Navalny’s personal doctors have reported that he is suffering from a range of severe symptoms that they call life threatening.
There was no immediate response from Mr. Navalny’s political allies or personal doctors about the recommendation for treatment with vitamins. Over the weekend, they said that Mr. Navalny’s blood tests had showed a risk of imminent heart or kidney failure.
His potassium levels were elevated, and tests showed other signs of possible kidney ailments, his doctors said. But starvation is only one issue in his declining health. Mr. Navalny’s lawyers say he may also be suffering from the lingering effects of a near-fatal poisoning with a military nerve agent last summer.
Mr. Navalny was treated in Germany after the apparent poisoning, but upon his return to Russia, he was arrested on a parole violation for a conviction that he and his allies dismissed as politically motivated. He is currently serving a two-and-a-half-year sentence.
The United States and European governments issued statements demanding adequate treatment for Mr. Navalny, and the American national security adviser, Jake Sullivan, said the Russian government would face “consequences if Mr. Navalny dies.”
The transfer to a hospital in a high security prison east of Moscow could indicate a worsening of Mr. Navalny’s condition. But the statement from the prison authorities suggested that the goal was closer medical observation. “Presently, the health of A. Navalny is evaluated as satisfactory,” the statement noted. It added that he was being observed daily by a doctor and that he had agreed to begin a course of “vitamin therapy.”
Mr. Navalny, 44, who has been President Vladimir V. Putin’s main political opponent for more than a decade, had been in good health before the chemical weapon poisoning last summer. The nerve toxin and treatment left him in a coma for weeks.
In prison, he has reported a range of symptoms including back pain and loss of sensation in his legs and arms. Prison doctors said medical imaging found slipped disks in his back. He also had a high temperature and was coughing, though authorities said tests for Covid-19 and tuberculosis, a common disease of Russian prisons, had been negative.
Blood tests, though, showed signs of possible kidney failure that could cause a lethal irregularity in Mr. Navalny’s heartbeat, his personal doctor, Dr. Anastasia Vasilyeva, said over the weekend in a joint statement with specialist doctors.
The transfer to a prison hospital will not help Mr. Navalny, Ms. Vasilyeva said Monday. The site specializes in treating prisoners with tuberculosis, she noted. “This is absolutely not a hospital where they can diagnose and treat his problems,” she wrote in statement posted on social media.
The Kremlin spokesman, Dmitri S. Peskov, said on Monday that Mr. Putin was not monitoring Mr. Navalny’s health. Decisions on his care were left to the prison service, he said on a conference call with journalists.
“I also don’t have information about the health of this prisoner you mention,” he said.