Doctors for the jailed Russian opposition activist Alexei Navalny say he "will die within the next few days" if not given medical attention.

Axar.az reports that doctors say recent blood test results indicate he could suffer cardiac arrest or kidney failure at any moment.

Navalny has been on a hunger strike for 18 days, to demand proper treatment for acute back pain and leg numbness.

A prominent critic of President Vladimir Putin, he was jailed in February for old embezzlement charges.

Four doctors, including his personal physician, Anastasia Vasilyeva, have written to prison officials to ask for permission to see him urgently.

In the letter, which Dr Vasilyeva posted on Twitter, the experts said Alexei Navalny's potassium had reached "critical levels".