Russian prison officials are threatening to start force-feeding jailed Kremlin critic Alexei Navalny, his team said Monday, after he lost eight kilograms (18 pounds) since starting a hunger strike.

-Seeing the seriousness of the hunger strike, the administration is threatening every day to start force-feeding- Navalny's team said in a post on his Twitter account.

It reported that Navalny, who last week said he had a cough and fever, had been transferred back to the prison barracks from its infirmary.

-They are still not allowing a doctor to see him- it said.

The 44-year-old opposition politician now weighed just 77 kilograms (169 pounds), it said, down from 85 kilograms (187 pounds) when he started the hunger strike on March 31.

Navalny, who is 189 centimetres (six feet two inches) tall, had already lost significant weight in prison before launching the hunger strike. He weighed 93 kilograms (205 pounds) when he entered the facility in February.

The anti-corruption campaigner, who barely survived a poisoning with nerve agent Novichok last August, began refusing food in protest at what he said was a lack of proper medical treatment in prison for severe back pain and numbness in his legs.

President Vladimir Putin's best-known opponent, Navalny was arrested in mid-January when he returned to Russia from Germany, where he had been treated for the poisoning, and was sentenced to two and a half years in prison on old embezzlement charges in February.

Members of his defence team, who visited him in his penal colony in the town of Pokrov 100 kilometres (60 miles) east of Moscow last week, said he was also losing sensation in his hands.

'Innocent man tortured'

Lawyer Olga Mikhailova said that an MRT scan had shown that Navalny has two herniated discs in his back, as well as a bulging disc.

On Monday, Russians across the country were celebrating the 60th anniversary of Yuri Gagarin's historic first spaceflight, and Navalny's team urged Russians not to forget the opposition politician's plight despite the large-scale commemorations.

-Yes, Cosmonautics Day is cool, of course- ally Lyubov Sobol tweeted. -But an innocent man is being tortured in front of the whole country right now.-

Navalny's lawyers and allies are demanding that he be transferred to a regular hospital. The Kremlin has said that Navalny is not entitled to any special treatment.

Navalny has been a thorn in the Kremlin's side for a decade by probing corruption among officials and leading large protests throughout Russia supported by a regional network.

His team has launched a new campaign seeking his release and announced plans to stage what they said would be "modern Russia's biggest protest."

The team said they would set a date for the protest once 500,000 supporters had registered with a website. As of Monday, nearly 420,000 people had signed up.

The team, which faces regular searches and arrests by law enforcement, also opened a new office in the North Caucasus republic of Dagestan on Sunday.

But on Monday, the head of the regional network, Leonid Volkov, wrote on Twitter that "both of its employees have gone missing".