MOSCOW (NewsNation) — Russian opposition politician Alexei Navalny is unconscious and in intensive care after being poisoned, a spokeswoman said Thursday.

Navalny, 44, is on an artificial lung ventilator, his spokeswoman, Kira Yarmysh, said on social media. She believed it was a deliberate poisoning.

“We assume that Alexei was poisoned with something mixed into his tea. It was the only thing that he drank in the morning. Doctors say the toxin was absorbed faster through the hot liquid. Alexei is now unconscious,” Yarmysh said.

Navalny, a critic of President Vladimir Putin, had started feeling ill when returning to Moscow from Tomsk in Siberia by plane on Wednesday morning, she added.

He had tea at a cafe at Tomsk airport before boarding his flight, Reuters reported.

His plane later made an emergency landing in Omsk so that he could be rushed to hospital, Yarmysh said.

Yarmysh drew a parallel with an incident last year in which Navalny suffered an acute allergic reaction that one doctor said could have resulted from poisoning with an unknown chemical.

“Obviously the same has been done to him now,” said Yarmysh. She did not say who she believed may have poisoned Navalny, but said police had been called to the hospital.

Navalny, a lawyer and anti-corruption activist, has served several stints in jail in recent years for organizing anti-Kremlin protests.

He has helped release investigations into what he has said are examples of official corruption.

The European Court of Human Rights has ruled that Russia’s arrests and detention of Navalny in 2012 and 2014 were politically-motivated and violated his human rights, a ruling Moscow called questionable.