MOSCOW: Jailed Kremlin critic Alexei Navalny risks cardiac arrest at "any minute" as his health has rapidly deteriorated, doctors warned Saturday, urging immediate access to Russia's most famous prisoner.

On March 31, President Vladimir Putin's most prominent opponent went on hunger strike to demand proper medical treatment for back pain and numbness in his legs and hands.

On Saturday, US President Joe Biden added his voice to a growing international chorus of protest at the treatment of the activist, describing his situation as "totally unfair".

Navalny, 44, was imprisoned in February and is serving two-and-a-half years on old embezzlement charges in a penal colony in the town of Pokrov around 100 kilometres (60 miles) east of Moscow.

Navalny's personal doctor Anastasia Vasilyeva and three more doctors including cardiologist Yaroslav Ashikhmin have asked prison officials to grant them immediate access.

"Our patient can die any minute," Ashikhmin said on Facebook on Saturday, pointing to the opposition politician's high potassium levels and saying Navalny should be moved to intensive care.

"Fatal arrhythmia can develop any minute."

Navalny barely survived a poisoning with the Novichok nerve agent in August which he has blamed on the Kremlin. His doctors say his hunger strike might have exacerbated his condition.

Having blood potassium levels higher than 6.0 mmol (millimole) per litre usually requires immediate treatment. Navalny's were at 7.1, the doctors said.

"This means both impaired renal function and that serious heart rhythm problems can happen any minute," said a statement on Vasilyeva's Twitter account.

The doctors said he had to be examined immediately "taking into account the blood tests and his recent poisoning".