Jailed Kremlin critic Alexei Navalny on Wednesday said he is going on hunger strike until he receives proper medical treatment for severe back pain and numbness in his legs.

President Vladimir Putin's most prominent critic, who is serving a 2.5-year sentence in one of Russia's most notorious penal colonies, said he was losing sensation in both legs and demanding he be properly treated.

The 44-year-old said was suffering from a pinched nerve that had caused his right leg to go numb and accused prison officials of refusing to provide him with adequate medical treatment.

He complained he had only been given painkillers, but had not been properly diagnosed.

In a post on Instagram on Wednesday, Navalny said that the back pain was now causing a loss of feeling in his left leg, too.

"I have gone on a hunger strike demanding that the law be obeyed and that a doctor be allowed to visit me," he said.

Navalny, who is considered a flight risk by authorities, last week filed two formal complaints against prison authorities, saying that he is woken eight times a night by guards announcing to a recording camera that he is still in his cell.

 'Jokes aside'

On Wednesday, he said that instead of receiving medical treatment he is continuing to be "tortured through sleep deprivation".

"I have the right to ask for a doctor and receive medicine," he wrote. "Jokes aside but this is already bothering me."

Navalny was detained in mid-January after returning to Russia from Germany, where he had been recovering for several months from a poisoning attack with Novichok nerve agent he says was orchestrated by the Kremlin.

He spent February between detention centres and courtrooms in Moscow where he was on trial for slandering a World War II veteran and violating the terms of an old suspended sentence handed down for fraud.

Last week he said he believed he had suffered a pinched nerve from routinely being ferried in police wagons and standing "crookedly" in court cages for defendants.

He joked that he did not want to "part with" his right leg and quipped about becoming a one-legged pirate.

Navalny's allies and Western governments say his prosecution is politically motivated.

Rights activists say his penal colony is one of Russia's worst and his wife Yulia has appealed directly to Putin to set his critic free.

Europe's rights court had called on Russia to release the Kremlin critic out of a concern for his life, but Moscow rejected the call.