MOSCOW — Doctors treating Russian opposition leader Alexei Navalny have said they have not found any poison in his blood after the 44-year-old fell ill during a flight into Moscow.

Navalny's allies and supporters have claimed the opposition figure, who is in a coma and on a ventilator at a hospital in Omsk, has been poisoned because of his political activity.

On Friday, doctors denied permission for him to be transferred in Germany to seek medical treatment due to his "unstable" condition.

Navalny's spokeswoman, Kira Yarmysh, affirmed such ban "means a direct threat to his life" and was decided by "the Kremlin".

Navalny was meant to be flown to Germany by NGO Cinema for Peace. Their plane departed from Nuremberg overnight and was expected to land in Siberia on Friday morning.

The NGO, which is known for its actions helping human rights defenders in Russia, has already carried out a similar operation when it medevacked Piotr Verzilov, a member of controversial Russian group Pussy Riot, after he was poisoned in 2018.

Yarmysh said Navalny's personal doctor from Moscow, his wife Yulia Navalnaya and Ivan Zhdanov, chief of Navalny's anti-corruption foundation, flew to Omsk. Neither of them, said Yarmysh, was allowed to see Navalny.

His doctor Anastasia Vasilyeva asked for support from the Ministry of Health to gain access to his medical records and transport him to a hospital in Moscow or abroad.

Yarmysh alerted Twitter users on Thursday morning, saying Navalny is in intensive care in a Siberian hospital after apparently being poisoned.

Navalny was on a flight from Siberia to Moscow early on Thursday morning when the plane had to make an emergency landing due to a sudden deterioration of his health, his spokesperson, Kira Yarmysh tweeted. "Alexei has been poisoned, intoxicated," and "is now in intensive care."

She told the Echo Moskvy radio station that Navalny was sweating and asked her to talk to him so that he could “focus on the sound of a voice." He then went to the bathroom and lost consciousness.

S7 airlines sent a statement to Euronews on Thursday:

"Soon after take-off of the S7 2614 Tomsk – Moscow flight, the well-being of one of the passengers, Alexei Navalny, sharply deteriorated. The crew worked quickly and strictly in accordance with the procedures.

“The flight attendants immediately reported this to the captain, who made the decision to land at the nearest airport - in Omsk. Onboard Alexei did not eat or drink anything.

The ambulance took Alexey straight to intensive care. Two passengers flying with him also stayed in Omsk. After refueling the aircraft, the crew continued their flight to Moscow."

Vyacheslav Gimadi, a lawyer with Navalny's foundation, said the team is requesting Russia's Investigative Committee open a criminal probe. “There is no doubt that Navalny was poisoned because of his political stance and activity,” Gimadi said in a tweet on Thursday.

State news agency Tass confirmed that Navalny was admitted to the intensive care unit of the toxicology department in the emergency department of Omsk hospital in Siberia.

Yarmysh, who continues to document the events at the hospital, highlighted on Twitter that a large amount of police had arrived to investigate and posted a photo of a hallway filled with security staff.

Anatoliy Kalinichenko, deputy chief doctor of the hospital, told reporters that Navalny was in grave, yet stable condition. Kalinichenko said doctors are considering a variety of diagnoses, including poisoning, but refused to give details, citing a law preventing doctors from disclosing confidential patient information.

As more information about Navalny's condition comes to light, political leaders are apprehensively watching the situation unfold. MEP Guy Verhofstadt wished Navalny a quick recovery on Twitter.

Lithuania's Foreign Minister Linas Linkevicius tweeted he was concerned and following the events. French president Emmanuel Macron indictated France was ready to offer Navalny “all necessary assistance." He told reporters on Thursday that France was "extremely worried and saddened” by what happened, and that he had offered the opposition leader and his family help with medical care or other unspecified protection.

German Chancellor Angela Merkel. speaking at a joint news conference with Macron, said Germany also will insist on transparency regarding Navalny’s illness and expressed support for him.

“Obviously Germany will let him have all the medical help that is needed also in German hospitals,” Merkel said. “But that must of course be a wish expressed from there.”

Navalny's Foundation for Fighting Corruption has been exposing graft among government officials, including some at the highest level. Last month, the politician had to shut the foundation after a financially devastating lawsuit from Yevgeny Prigozhin, a businessman with close ties to the Kremlin. — Euronews