The European Court of Human Rights on Wednesday ordered Russia to "immediately" release jailed Kremlin critic Alexei Navalny, saying his life was in danger in prison.

Navalny, seen as the main domestic opponent of President Vladimir Putin, was arrested and jailed upon returning to Russia last month following months of treatment in Germany for a nerve agent poisoning.

"The Court decided to indicate to the Government of Russia... to release the applicant. This measure shall apply with immediate effect," the ECHR said.

It said that the ruling was taken with "regard to the nature and extent of risk to the applicant's life".

Russia is a member of the Council of Europe rights body, which the ECHR belongs to. Member states are obliged to enforce ECHR decisions, though in practice they may not be implemented. 

The court said Russia insisted in its argument to the court that Navalny was being held in an appropriately guarded facility and that his cell was under video surveillance.

But Navalny, who lodged a complaint on January 20, contended that "the arrangements listed by the government could not provide sufficient safeguards for his life and health," the court said. 

The 44-year-old opposition leader was sentenced to nearly three years in jail after he returned from Berlin, in a verdict that sparked international outcry.

The West already suspects that the Kremlin was behind Navalny's poisoning over the summer with the nerve agent Novichok, which made him fall gravely ill on a flight in Siberia.

Russia's penitentiary service accused Navalny of breaking the conditions of a suspended sentence for fraud by not checking in with authorities while he was recovering from the nerve agent poisoning.