Russia opposition leader Alexei Navalny is receiving intensive care at a Siberian hospital after he fainted on a plane. Navalny's spokeswoman Kira Yarmysh suggested he was poisoned by contaminated tea.

The 44-year-old fell ill on a flight back to Moscow from the Siberian city of Tomsk on Thursday and was taken to a hospital after the plane made an emergency landing in Omsk, Yarmysh said on Twitter. Navalny only drank tea in the morning, she said, before he became sick.

The regional health ministry department said during a press conference that Navalny is currently "in a natural coma," according to news agency TASS. And "he is still on an artificial lung ventilation machine." 

Tass reported that police were not considering deliberate poisoning.

According to Dr Anatoly Kalinichenko, deputy chief doctor of the Omsk hospital of intensive care where Navalny is hospitalized, doctors have ruled out a heart attack, stroke or that he contracted coronavirus.

"There is no data on traumatic injuries, including traumatic brain injury," Kalinichenko said. "There is no evidence of stroke or acute myocardial infarction... we didn't find any COVID infections," he said.

Navalny's organization was scrambling to make arrangements to transfer him to Germany for treatment. A German group said it was ready to send a plane for him and that a noted hospital in Berlin was ready to admit him.

Omsk is about 4,200 kilometers east of Berlin, roughly a six-hour flight.

In a video statement released early Friday in Omsk, Yarmysh said Navalny remained in critical condition and she called on the hospital's leadership "not to obstruct us from providing all necessary documents for his transfer." It was not clear what the possible obstructions could be.

President Vladimir Putin's spokesman Dmitry Peskov said that "if the Kremlin or the Russian Health Ministry received a request for Navalny's transfer to a foreign hospital, it would be considered promptly."

Meanwhile, Peskov has pledged that authorities will launch an investigation if it emerges Navalny was poisoned. The statement came as S7 airlines revealed that the activist did not eat or drink anything on board his scheduled flight from Siberia to the Russian capital.

(With input from agencies)