Alexei Navalny
Alexei NavalnyReuters

Russia's penitentiary service said on Monday it was transferring ailing Kremlin critic Alexei Navalny to a prison hospital, after the EU and the US warned they would hold Moscow responsible for the state of his health, AFP reports.

Russia's prison service, which has barred Navalny's own medical team from visiting him, said its doctors had decided to move him to a medical facility at another penal colony outside Moscow.

It insisted his condition was "satisfactory", and said he was taking vitamin supplements as part of medical treatment.

Navalny, one of Russian President Vladimir Putin's most prominent critics, was arrested on January 17 for alleged parole violations after returning from Germany, where he had been recovering from being poisoned with a military-grade nerve agent.

He was later sentenced by a Russian court to a three-and-a-half-year sentence, though his lawyer said he would serve only two years and eight months in jail because of time he has already spent under house arrest.

In late March, Navalny launched a hunger strike to protest the authorities’ failure to provide proper treatment for his back and leg pains.

Earlier this month, he was moved to a medical facility to be treated for a possible respiratory illness.

On Saturday, Navalny's doctors warned that he could die "any minute", pointing to the opposition politician's high potassium levels and saying Navalny should be moved to intensive care.

The United States on Sunday threatened Russia with "consequences" if Navalny dies in jail.