Russia has released a former US Marine who was jailed in 2022 and was reportedly in extremely poor health.

Robert Gilman, from Massachusetts, was freed "from detention for humanitarian reasons" and flying on a US plane to Washington, DC, on Tuesday, according to Kirill Dmitriev, special envoy for Russian President Vladimir Putin.

Massachusetts Senator Ed Markey posted on social media that Gilman had been "essentially catatonic" and battling a "severe case of pneumonia" before he was released.

Gilman was 28 when he was arrested for kicking a police officer while drunk aboard a train to Moscow.

"Approximately 50 days ago, Robert was diagnosed with a dissociative stupor - a condition where he is non-responsive and unable to interact or eat," Markey said, adding that the details of Gilman's current condition were unknown.

Reuters reported that multiple doctors were tending to Gilman as he returned to the US and that a US official said "he seems to be in good shape".

His family is awaiting his arrival at a military hospital in Texas, according to CBS News, the BBC's US partner.

"I am so grateful to President Trump for saving my brother's life," Lexie Hudson, his sister, said in a statement. "There is no other reason that Robert is alive today other than that President Trump heard about the case and took action."

-BBC