The US has warned Russia “there will be consequences” if Kremlin critic Alexei Navalny dies.
Vladimir Putin nemesis Navalny, 44, is in a “critical” condition while on hunger strike in jail.
The opposition leader fell gravely ill after being poisoned with Novichok nerve agent on a flight to Moscow last August.
He was jailed on his return to Russia and is now said to be at risk of kidney failure and going blind two weeks into his hunger strike.
President Joe Biden’s national security adviser Jake Sullivan on Sunday said the US government has told Russia “there will be consequences” if Kremlin critic Alexei Navalny dies in prison.
He told CNN: “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.
“In terms of the specific measures that we would undertake, we are looking at a variety of different costs that we would impose and I am not going to telegraph that publicly at this point, but we have communicated that there will be consequences if Mr Navalny dies.”
Navalny started refusing food on March 31 to protest a lack of proper medical treatment for his leg and back pain, said a medical trade union with ties to the opposition politician on Saturday.
“His condition is indeed critical,” said Alexandra Zakharova, a representative of the Doctors Alliance trade union – a group that Russian authorities regard as opposition activists.
She cited tests obtained through Navalny’s lawyers, saying members of the union had not examined him themselves.
“We have seen the tests, and they are very, very bad,” she said.
“His potassium is high and he has other high readings which indicate that his kidneys may soon fail. This would lead to severe pathology and cardiac arrest may occur,” she added.
Russia jailed Navalny for two-and-a-half years in February for parole violations he said were trumped up. He was arrested at the border as he returned to Russia from Germany where he had been recovering from a nerve agent poisoning.
About 80 famous writers, actors, historians, journalists and directors, including authors JK Rowling and Salman Rushdie, wrote an open letter to Putin on Friday, urging him to ensure Navalny gets the urgent medical care he needs.
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Prison authorities say they have offered Navalny proper treatment, but that he has refused it and insists he should be treated by a doctor of his choice from outside the facility, a request they have declined.
Navalny said on Friday that prison authorities had threatened to put him in a straitjacket to force-feed him unless he abandons his hunger strike.