Kremlin critic Alexei Navalny, on a hunger strike in prison, has been threatened with being force-fed, according to his official Twitter account.
The 44-year-old announced last month he was going to begin refusing food in a bid to get proper medical treatment.
Navalny -- the highest-profile critic of Russia's president, Vladimir Putin -- was jailed earlier this year on fraud charges.
Tweets from his account on Monday said he had lost 8kg since the start of his hunger strike, and 15kg since the start of his prison sentence, in which time he has repeatedly raised concerns about his physical welfare.
"Seeing the severity of the hunger strike, the administration threatens to force-feed every day," said another tweet. "At the same time, Alexei was transferred back from the medical unit. The doctor is not allowed to see him."
Prior to the hunger strike, Navalny had complained of severe back pain that has spread to one leg and says his other leg is numb.
He has also accused prison guards of torture by sleep deprivation, with his jailers waking him up every hour during the night.
Navalny was arrested in January upon his return from Germany, where he spent five months recovering from a Novichok chemical weapons attack that he blames on Russia.
The Russian government denies the accusations.
Last month, Navalny was sentenced to two and a half years in prison for violating the terms of his probation during his convalescence in Germany.
The sentence stems from a 2014 embezzlement conviction that Navalny has rejected as fabricated — and which the European Сourt of Human Rights has ruled to be unlawful.
Navalny was moved this month from a Moscow jail to a penal colony in the Vladimir region, 85 kilometres (50 miles) east of the Russian capital.