Jailed Kremlin critic Alexei Navalny said Friday he was suffering from nerve pain in a notorious Russian penal colony and said getting sick in prison was potentially fatal.

President Vladimir Putin's most prominent foe was last month sent to prison for two and a half years on old embezzlement charges his allies and Western government said were politically motivated.

His lawyers this week raised concerns about his health and called on prison authorities to clarify his condition and move him to Moscow for treatment.

"Apparently a nerve was pinched from constantly sitting in police wagons and in 'pencil cases' crookedly," Navalny said in a message posted on Instagram on Friday, referring to court cages for defendants.

"A week ago, the prison doctor examined me and prescribed two ibuprofen pills but I still don't know the diagnosis," he added.

Navalny, 44, was detained in January after returning to Russia from Germany where he had been recovering for several months from a poisoning attack 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.

Russian opposition leader Alexei Navalny's lawyers have raised concerns about his health in prison
Russian opposition leader Alexei Navalny's lawyers have raised concerns about his health in prison AFP / Kirill KUDRYAVTSEV

He said on Friday that Mikhail Khodorkovsky -- who owned the former oil giant Yukos before he was convicted in two controversial cases and spent a decade behind bars -- had warned him that getting sick in Russian prison was a potential death sentence.

"Once Mikhail Khodorkovsky, who served 10 years in prison, told me: 'The main thing is not to get sick there.'"

"Nobody will treat you. If you get seriously ill, you will die," Navalny said Khodorkovsky had told him.

Navalny on Thursday filed two formal complaints, saying he was being subjected to sleep deprivation that amounted to "torture".

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

The Kremlin, which denied involvement in Navalny's poisoning, said questions about his health should be directed to the prison service.

On Thursday, Navalny's wife Yulia appealed directly to Putin to set his critic free.