Russian opposition leader and outspoken Kremlin critic Alexey Navalny was unconscious and on a ventilator in a Siberian hospital 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 Alexey 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 said.

Navalny has been admitted to the Omsk emergency hospital No. 1 and is in a "serious condition," hospital head physician Alexander Murakhovsky said, according to Russian state news agency TASS.

Russian state media RIA Novosti, citing hospital staff, reported that Navalny was in an emergency unit and that his condition was "serious, stable.

"The intensive care unit is full of police officers, Yarmysh tweeted."They try to get an explanation from the doctor.

The doctor saw me in the distance in the corridor, said that 'some things are confidential' and took the police to another room," Yarmysh said.

"The evasive reaction of doctors only confirms that this is poisoning," Yarmysh added.

Уклончивая реакция врачей только подтверждает, что это отравление. Ещё два часа назад они были готовы поделиться любой информацией, а теперь явно тянут время и не говорят, что им известно

— Кира Ярмыш (@Kira_Yarmysh) August 20, 2020

"Just two hours ago, they were ready to share any information, and now they are clearly biding for time and are not saying what they know.

"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.