MOSCOW: Jailed Kremlin critic Alexei Navalny on Monday said he was suffering from a heavy cough and fever but would continue a hunger strike he launched last week demanding adequate medical treatment.

Navalny, 44, announced the hunger strike on Wednesday, complaining a prison doctor had only given him painkillers as treatment for severe back pain and numbness in his legs.

His team the next day said he had already lost eight kilogrammes (18 pounds) before going on hunger strike -- down from the 93 kilogrammes (205 pounds) he weighed when he arrived at his penal colony -- due to sleep deprivation.

In a post on his Instagram Monday, Navalny reported new ailments.

"I am quoting the official data from today's temperature measurement: 'Navalny A.A., strong cough, temperature 38.1'," he wrote, referring to degrees Celsius, or 100.6 degrees Fahrenheit.

"P.S. I am continuing my hunger strike, of course," Navalny said.

The opposition politician is serving a two-and-a-half year sentence on old fraud charges in a penal colony some 100 kilometres (62 miles) east of Moscow known for its harsh discipline.

His latest statement from the colony comes after pro-Kremlin media on Friday launched an offensive aiming to disprove his complaints of mistreatment and lack of medical attention.

Two reports in pro-Kremlin outlets described Navalny as looking "quite normal" and saying he is incarcerated in a colony that is "practically exemplary".

In his post Monday, Navalny said the reports had "not a single word of truth".

As evidence, he wrote that a third person out of 15 total in his unit had been hospitalised with tuberculosis since his arrival at the penal colony in February.

"I am surprised that there is no Ebola virus here," he quipped, adding: "such is our 'ideal, exemplary colony'."

Navalny was arrested on his return to Russia in January, after spending months in Germany recovering from a poisoning last summer that he blames on the Kremlin.