Originally Published: 12 APR 23 12:27 ET

By Jack Guy, Darya Tarasova and Xiaofei Xu, CNN

(CNN) -- Jailed Russian opposition figure Alexey Navalny has been experiencing severe stomach issues in prison, and members of his team fear that he may have been poisoned again.

Navalny lost eight kilograms over the past 15 days in his isolated prison cell, his spokesperson Kira Yarmysh said in a Twitter post on Tuesday.

"The lawyer says that an ambulance was called for Alexey Navalny on the night of Friday to Saturday because of an acute stomach pain. No one is treating him and they are not even telling him the diagnosis," Yarmysh said.

"We do not rule out that all this time in prison he could have been poisoned with something to make his health deteriorate slowly but steadily," she added.

Navalny's lawyer Vadim Kobzev said said that his team will request toxicological and radiological tests.

"This may sound like nonsense and paranoia to someone else, but not to Navalny after Novichok," Kobzev tweeted on Tuesday.

Novichok is the nerve agent which was used to poison Navalny in 2020, according to the German government, an act he and Western officials blame on the Kremlin.

After recovering, just a few months later he returned to Russia where he was soon detained.

Navalny faces additional challenges in the prison, including threats from fellow inmates, according to Yarmysh and Kobzev, who appealed to the public to help fight for his release.

CNN has reached out to Russia's federal penitentiary service (FSIN) for comment.

Navalny is the best-known opposition politician in Russia. Even while behind bars, his Instagram and Twitter accounts keep up his attacks on Russian President Vladimir Putin as he passes notes for his team to post online.

Investigative journalist Christo Grozev, who worked on CNN's "Navalny" documentary, said he believes Putin has "motivation" to poison Navalny.

"I cannot exclude that there is the motivation for that. We know that Navalny was definitely poisoned with Novichok by none other than the state of Russia. We know that he went back to Russia against the wishes of Putin," Grozev told CNN's Erica Hill on Tuesday.

"And, of course, now from within jail, Navalny continues to essentially ridicule Putin in the war effort by being a very vocal opponent of the war. So of course there is the motivation," he said.