MOSCOW — After his allies warned that he is "days" away from dying, jailed opposition leader Alexei Navalny has been moved to a prison hospital, Russia's federal penitentiary service said Monday.

Navalny is 21 days into a hunger strike, demanding access to proper medical care at his expense. His attorneys said Navalny, 44, has two herniated discs that have caused extreme back pain and numbness in his leg and hands.

In social media posts, doctors affiliated with Navalny said his health has rapidly deteriorated because of the hunger strike and possible lingering effects from the nerve-agent poisoning that nearly killed him in August.

Yaroslav Ashikhmin, Navalny’s personal physician, said that blood tests revealing high potassium were indicative of kidney failure and that severe heart-rhythm disturbances threatened cardiac arrest.

“Our patient could die at any moment,” Ashikhmin said on Facebook.

Anastasiya Vasilyeva, Alexei Navalny’s physician, said she and other doctors waited for two hours but were not allowed to see the opposition leader. (Anastasiya Vasilyeva's Team via AP)

The federal penitentiary service said Monday: “At the present time, Navalny’s health is assessed as satisfactory, and he is being examined daily by a physician.”

“With the patient’s consent, he was prescribed vitamin therapy,” the statement added.

The penitentiary service said Navalny was transferred from Penal Colony No. 2, about 60 miles east of Moscow, to a special unit of the regional hospital in Penal Colony No. 3, which “specializes in the dynamic observation of such patients.”

Navalny’s Twitter account said Monday that the news of his transfer to a hospital was “for loud headlines” and that Penal Colony No. 3’s hospital specializes in treating tuberculosis patients. He has no access to the account, but posts are made by his team.

“Alexei is in a colony,” the account said. “Doctors are still not allowed to see him.”

Anastasia Vasilyeva, a Navalny ally who heads a small independent medical workers’ union, said on Twitter that moving Navalny, who does not have tuberculosis, to an area with tuberculosis patients is “a decision to forcibly infect a person with a reduced immune status with tuberculosis, just to prevent his doctors from seeing him.”

U.S. national security adviser Jake Sullivan told CNN on Sunday that the Biden administration has warned Russia that there will be consequences if Navalny dies in prison.

In an interview with the BBC broadcast on Sunday, Russia’s ambassador to the United Kingdom, Andrei Kelin, said Navalny “will not be allowed to die in prison” and claimed that his calls for medical help are an attempt to “attract attention.”

Navalny was poisoned with a chemical weapon in the same class as the Soviet-era nerve agent Novichok while traveling from Siberia in August. U.S. and European authorities have blamed the attack on Russian state agents and have responded with sanctions against Russian organizations and individuals.

After recovering in Germany from the nerve-agent attack, Navalny returned to Russia in January, and he was immediately arrested on charges of violating the terms of his parole from a 2014 embezzlement case. Navalny and his allies have denounced the case as trumped-up to silence him. He was sentenced to more than two years in prison.

Navalny’s team on Sunday announced new protests in cities across Russia for Wednesday, the same day President Vladimir Putin is set to deliver an annual address in parliament. Navalny has said that Putin wants him dead.

“Have you ever seen with your own eyes how they kill a person?” his representatives asked in a statement. “You are seeing it right now. . . . They are killing Alexei Navalny. In a terrible way. In front of us all.”

Warning citizens against attending “unsanctioned actions,” Russia’s Interior Ministry said Monday that “law enforcement agencies will prevent any destabilization of the situation and will take all appropriate measures to maintain law and order in the country’s regions.”

“Any aggressive actions by participants in unsanctioned public events, especially any attempts at provoking clashes with law enforcement officers, will be regarded as public security threats and suppressed immediately,” the ministry said.

More than 11,000 protesters were reported detained across Russia in pro-Navalny rallies in January and early February, according to the independent police watchdog group OVD-Info.

Prosecutors have requested that Navalny’s network of regional offices and his Anti-Corruption Foundation be added to a list of “terrorist and extremist” organizations, clearing the way for the organizations to be banned in Russia, with possible jail time for their members.