Kremlin critic Alexei Navalny risks cardiac arrest "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 -- who was imprisoned in February -- went on hunger strike to demand proper medical treatment for back pain and numbness in his legs and hands.
Navalny 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 a cardiologist, have asked prison officials to grant them immediate access.
Their letter to the head of Russia's Federal Prison Service was posted on Vasilyeva's Twitter account on Saturday.
A statement on her Twitter account said his potassium had reached "critical levels".
"This means both impaired renal function and that serious heart rhythm problems can happen any minute," it added.
Having blood potassium levels higher than 6.0 mmol (millimole) per litre usually requires immediate treatment. Navalny's were at 7.1, the statement said.
In the letter to the prisons chief, the doctors said Navalny, who was critically ill after a poisoning with the Novichok nerve agent, could suffer "cardiac arrest".
They said he had to be examined immediately "taking into account the blood tests and his recent poisoning".