Navalny, 44, a prominent opponent of Russian President Vladimir Putin, announced a hunger strike at the end of March in protest at what he said was the refusal of prison authorities to treat him properly for acute back and leg pain.

Prison authorities say they have offered him proper treatment but that he has refused it and insists he should be treated by a doctor of his choice from outside the facility, a request they have declined.

In a post on his Instagram page, which his allies use to provide updates based on information from his lawyers who regularly visit him, Navalny said that he was experiencing dizziness but that he was able to walk.

"There was a female colonel standing over me this morning saying: your blood tests show a serious deterioration in your heath and (that there is) a risk. If you don't give up the hunger-strike, we're ready to start force-feeding you now," he wrote.

"She went on to tell me about the delights of force-feeding. A straitjacket and other joys," he said.

Russia jailed Navalny for two-and-a-half years in February for parole violations he said were trumped up.

He was arrested at the border as he returned to Russia from Germany where he had been recovering from a nerve agent poisoning.

An outspoken Putin critic for years, Navalny has organised anti-Kremlin street protests and carved out a following online with investigations alleging corruption by senior Russian officials.

The local prison service in Vladimir region, where the IK-2 facility holding him is located about 100km east of Moscow, did not reply to a written request to comment.

Australian Associated Press