Russian opposition leader and outspoken Kremlin critic Alexei Navalny was placed on a ventilator and was unconscious in a hospital in Siberia Thursday after falling ill from suspected poisoning, his spokesperson said.

Navalny, 44, started feeling unwell while on a return flight to Moscow from the Siberian city of Tomsk, his spokeswoman Kira Yarmysh, said on Twitter. The plane later made an urgent landing in Omsk, she added.

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

Navalny remains unconscious and is now connected to a ventilator, Yarmysh added.

Citing hospital staff, Russian state media RIA Novosti reported Navalny was in an emergency unit and that his condition is “serious, stable.”

Navalny previously suggested he might have been poisoned in July last year, while he was being held in police custody and suffered a mysterious allergic reaction.

This story has been updated to correct the spelling of Navalny's spokesperson.