Jailed Kremlin critic Alexei Navalny said Monday he had received nearly a dozen warnings for his behaviour since being imprisoned last month and risks time in solitary confinement.

President Vladimir Putin's most prominent critic is serving a two-and-a-half-year sentence on old embezzlement charges Navalny says are politically motivated.

His allies and rights activists say his penal colony is one of Russia's worst and have raised concerns about his health.

Navalny, 44, said that in the past four weeks he had received 10 formal reprimands from prison authorities, including six from guards at his current prison in the town of Pokrov outside the Russian capital.

-If you receive two reprimands you can go to a punishment cell and that's unpleasant, conditions there are close to torture- he said in a post on Instagram.

He said prison officials were currently considering more warnings against him.

His transgressions include getting up before the allotted time, refusing to watch an "idiotic" video lecture and suggesting to the head of his prison unit to "have coffee" instead of doing exercises.

Navalny said last week he was suffering from severe back pain and numbness in his legs and complained medical staff had neglected to properly treat him.

Since being transferred to Penal Colony No. 2 in Pokrov, Navalny has also issued several light-hearted statements likening his prison routine to that of a stormtrooper in the Star Wars films.

On Monday, he joked that the soundtrack to his prison stay was "Bad Guy", a hit by American pop singer Billie Eilish.

Navalny was arrested on his return to Russia in mid-January from Germany, where he spent months recovering from a near-fatal nerve agent poisoning in August that he blamed on the Kremlin.