Jailed Russian opposition politician Alexei Navalny says he is suffering worsening back pain from long spells in solitary confinement that he says are part of a deliberate strategy by the authorities to undermine his health.

In a post on Twitter, he also complained of being injected with unknown drugs.

"See how the system works when you are not allowed to beat up a person, but your leadership ordered you to hurt them badly," the Twitter post said.

"For example, I have a problem with my spine. It is clear what one has to do to make the problem worse: keep me immobile as much as possible," said Navalny, 46, who is able to post on social media via his lawyers and allies.

"If you lock a person up in a punishment cell, where he can either stand or sit on an iron stool for 16 hours a day, after a month in such conditions even a healthy person will undoubtedly get back pains.

"I've spent the last three months like this. Naturally, my back hurts a lot."

Mr Navalny said he had asked for a month and a half to see a doctor. When a doctor finally came, he said, she examined him for just five minutes and refused to tell him her diagnosis or what she was prescribing.

He said he had then been given injections.

When he asked what they contained, he was told: "We inject what the doctor prescribed. Vitamin B, for example."

The injections are not helping, he said, "and in general, I feel a little uncomfortable being injected with an unknown drug".

Reuters requested comment from the federal prisons service responsible for the penal colony east of Moscow where Mr Navalny is being held. There was no immediate reply.

The Twitter post included images from Mr Navalny's medical records, which he said had been released to him a month after he had asked to see them.

The hand-written sheets were hard to decipher in the form in which they appeared online.