Moscow(CNN) Nearly a dozen Russian politicians have published an open letter to President Vladimir Putin, saying that he was personally responsible for the life of Kremlin critic Alexey Navalny, who has been on a hunger strike in a penal colony in the Russian city of Pokrov.
"The state of health of political prisoner Alexey Navalny threatens his life," said the letter, signed by 11 politicians representing several regional parliaments, as they demanded an independent doctor is immediately allowed to visit Navalny.
The opposition figure went on a hunger strike on March 31, demanding "proper medical care" and to be examined by an independent doctor -- something his team claims he has been unable to get in the penal colony in Pokrov.
His allies have been raising concerns about the jailed Kremlin critic's health, saying Navalny "is dying" and his doctors said medical tests showed he's at growing risk of renal failure and heart problems. CNN is unable to independently verify the state of Navalny's health.
"We regard what is happening in relation to Navalny as an attempt on the life of a politician, committed out of personal and political hatred," the letter, which is open for Russian citizens to sign, said. It was first shared and circulated late Saturday night.
"You, the President of the Russian Federation, personally bear responsibility for the life of Alexey Navalny on the territory of the Russian Federation, including in prison facilities - [you bear this responsibility] to Navalny himself, to his relatives, and to the whole world." it said, while also calling for a review and cancellation of all Navalny's criminal cases.
The escalating situation has led Navalny's allies to announce rallies across Russia on April 21 in support of him.
"Navalny is now in the colony, his life hanging on a thread," Leonid Volkov, chief of Navalny's staff, said in a joint video with Ivan Zhdanov, the director of Alexey Navalny's anti-corruption foundation (FBK).
"He has been on a hunger strike for several weeks now, demanding medical attention. His condition is critical, and we do not know how much longer he can hold out. But it is clear that we do not have time," he added in the video, which called on citizens across Russia to take to local central squares.
Western countries have also condemned Russian authorities for allowing Navalny's condition to deteriorate. On Sunday, US national security adviser Jake Sullivan said the Biden administration is weighing options to punish Russia if Navalny dies in state custody.
"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," Sullivan told CNN. "In terms of the specific measures that we would take, 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."
"But we have communicated that there will be consequences if Mr. Navalny dies," he added.
The European Union called on Russia to grant opposition leader Alexey Navalny "immediate access" to a medical professional "he trusts" on Sunday.
"The European Union is deeply concerned about reports that the Russian opposition politician Alexei Navalny's health in the penal colony continues to deteriorate even further. We call on the Russian authorities to grant him immediate access to medical professionals he trusts," The EU's High Representative for Foreign Affairs Josep Borrell said in a statement.
Russian ambassador to the UK Andrey Kelin told the BBC in an interview aired Sunday that Navalny is "behaving like a hooligan," but could have a chance at being released from prison early if he "behaves normally."
He refused to elaborate on what would constitute normal behavior.
When pressed by the BBC to answer whether the Russian government was prepared to let Navalny die in prison, Kelin said: "of course he will not be allowed to die in prison. But I can say that Mr Navalny behaves like a hooligan, absolutely, in trying to violate every rule that has been established. His public purpose through all of that is to attract attention, also by saying that today his left hand is sick, tomorrow his leg is sick and all of that stuff."
Addressing critics that say Navalny is not allowed to see his personal doctor, Kelin asked: "How about British prisoners? Do British prisoners request personal doctors?"
"[Navalny] has got the necessary medical treatment and believe me, we will take care of his medical treatment," he added.
France's Foreign Minister said on Sunday his country was "extremely worried" about Navalny's health.
"I consider Mr. Navalny's situation extremely worrying," Jean-Yves Le Drian told public broadcaster France 3. "I note that there is really an authoritarian drift in Russia. The case of Mr. Navalny is the most symbolic, the most striking for the minds," he added.