U.S. Warns ‘There Will Be Consequences’ If Russian Dissident Alexei Navalny Dies

KIRILL KUDRYAVTSEV/AFP via Getty Aleksey Navalny (center) at a demonstration in Moscow in 2020

There will be "consequences" if Russian opposition leader Alexei Navalny dies in prison, a top U.S. official warned on Sunday.

Navalny, 44, is Russian President Vladimir Putin's most notable critic and has accused the Kremlin of attempting to assassinate him with a nerve agent and subsequently jailing him on politically motivated charges.

The Russian government denies this.

Navalny is being held in prison on a two-and-a-half year fraud sentence and has gone on a three-week hunger strike to demand he receive medical care from his own doctors — after back pain and loss of feeling in his legs — while officials say he is already getting adequate treatment.

He was detained for what Russian officials said was him breaking probation by being hospitalized in Germany after surviving a poisoning last year.

Most recently, he was moved to a prison hospital as his condition deteriorated amid his hunger strike, according to the Associated Press.

"Our patient could die at any moment," his doctor said on social media on Saturday.

His case has sparked widespread protests across Russia in recent months and he has gained worldwide support from human rights groups and activists, including George and Amal Clooney's Clooney Foundation for Justice.

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DIMITAR DILKOFF/AFP via Getty Aleksey Navalny

More than 70 prominent artists, writers, actors and other entertainers from around the world signed an open letter published in The Economist last week, demanding Navalny receive proper medical attention.

Among those who signed the letter addressed to Putin, 68, were stars such as Jude Law, Benedict Cumberbatch, J.K. Rowling, J.J. Abrams and Ian McKellen.

Putin has denied playing a role in Navalny's 2020 poisoning, while President Joe Biden — who has called the Russian president a "killer" — said earlier this year he had "raised concern" with Putin over Navalny.

On Sunday, National Security AdvisorJake Sullivan told CNN's State of the Union that "we have communicated to the Russian government that what happens to Mr. Navalny in their custody is their responsibility and they will be held accountable by the international community."

What steps the U.S. would take against Russia if Navalny dies remain unclear, however.

Putin's government is already a major focus of U.S. foreign policy. The White House has previously imposed sanctions and other isolating measures, while the Kremlin maintains America is choosing to fray relations between the two countries.

"We are looking at a variety of different costs that we would impose and I'm not going to telegraph that publicly at this point," Sullivan said Sunday before adding, "We have communicated that there will be consequences if Mr. Navalny dies."

Florian Gaertner/Photothek via Getty Vladimir Putin

Leaders around the globe have called for Navalny's release or, at the least, for him to receive the medical care he seeks.

"I am deeply worried about Alexei Navalny's health," European Commission President Ursula von der Leyen tweeted. "He must immediately receive access to proper medical treatment."

Biden said this weekend that he believed Navalny's treatment has been "totally unfair and totally inappropriate."

RELATED: Biden Says Putin Is a 'Killer' Who Will 'Pay a Price' for Election Interference

Navalny's doctors released a statement on Friday saying he "will die within the next few days," if he's not given adequate medical care.

Potassium in his blood levels, they wrote, were at "critical levels."

Human rights groups have said the prison where Navalny is being held is known for exceptionally harsh conditions, according to the BBC, which also reports that Amnesty International has described his treatment in prison as a slow torture.

According to the BBC, there is only one paramedic and no doctors assigned to the prison where Navalny is being held.

Russian officials have said they won't let Navalny die in prison.

"Russian authorities are responsible for Aleksey Navalny's well-being," Ned Price, a spokesman for the State Department, tweeted Sunday. "We call on them to allow him access to necessary medical care immediately in response to disturbing reports his health is deteriorating."