Jailed Kremlin critic Alexei Navalny's wife, Yulia Navalnaya, said on Tuesday she was increasingly concerned for her hunger-striking husband's health after visiting him at his penal colony outside Moscow.

Navalny, who is serving a two-and-a-half year sentence on old embezzlement charges, was jailed in February after returning to Russia from Germany where he was receiving treatment for a poisoning attack he says was orchestrated by the Kremlin.

Russia's most prominent opposition figure announced a hunger strike two weeks ago to demand adequate medical treatment, and his allies said this week that authorities had threatened to force feed him.

In a post on Instagram, Navalnaya said she had visited Navalny on Tuesday and communicated with him by phone and through a glass screen.

"He is just as cheerful and fun. But he speaks with difficulty and from time to time hangs up and lies down on the table to rest," she wrote.

"I know that he is not going to give up... But after the visit with Alexei, I worry about him even more."

She added that prison officials were continuing to prevent a doctor from seeing Navalny and that he now weighed 76 kilograms (168 pounds) -- down nine kilograms (20 pounds) since beginning his hunger strike almost two weeks ago.

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 and was down to 85 kilograms (187 pounds) when he started the hunger strike on March 31.

In his own Instagram post earlier Tuesday, the 44-year-old opposition politician said he was suing prison officials for denying him access to the Muslim holy book, the Koran.

Navalny said he was taking legal action against prison authorities because "they won't give me my Koran. And it's infuriating."

"When I was jailed, I made a list of ways I wanted to improve myself that I will try to complete in jail. One of the points was to deeply study and understand the Koran," he wrote.

"Books are our everything, and if you have to sue for the right to read, I will sue."

Navalny came under fire early in his political career for making nationalistic comments and deriding immigrants in Russia from predominantly Muslim countries in Central Asia.

His post came as many Muslims around the world started Ramadan after religious leaders confirmed the month of fasting would begin on Tuesday.

Navalny said he had read the Koran before but had not internalised its core tenets.

"I realised that my development as a Christian also requires studying the Koran," he wrote in the Instagram post.

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.

jbr-emg/dl