Doctors treating Russian opposition figure Alexei Navalny said Friday there was no evidence to back claims he was poisoned, as aides accused authorities of risking his life by refusing to allow his evacuation to Germany for treatment.

Navalny, a 44-year-old lawyer and anti-corruption campaigner who is among President Vladimir Putin’s fiercest critics, was in a coma in intensive care in the Siberian city of Omsk after he lost consciousness while on a flight and his plane made an emergency landing on Thursday.

Aides say they believe he was poisoned and that something was put in his tea at an airport cafe.

Doctors treating him in Omsk said Friday that tests had shown no trace of any poison and that Navalny was in no condition to be moved, despite the arrival of an air ambulance sent to take him to Germany.

“So far no poison has been identified in the blood and urine, there is no trace of its presence,” Anatoly Kalinichenko, the deputy head doctor of the hospital, told journalists in Omsk.

“We do not believe that the patient suffered poisoning,” he said, adding that doctors “practically” had a full diagnosis that had been communicated to Navalny’s family.

Kalinichenko said Navalny’s condition was “unstable”, making it too dangerous to move him.

Navalny’s spokeswoman Kira Yarmysh said the refusal was a ploy to hide the poisoning and put his life at risk.

“The ban on the transportation of Navalny is needed only to play for time until the poison in his body can no longer be traced,” she said on Twitter.

“Every hour of delay creates a critical threat to his life.”

Navalny’s team said earlier that the hospital in Omsk was ill-equipped and his doctor, Anastasia Vasilyeva, said she had asked for the Kremlin’s help to transfer him to a European clinic.

An air ambulance dispatched by a German charity to bring Navalny to Berlin for treatment had landed in Omsk, after Chancellor Angela Merkel extended an offer of treatment.

Foreign leaders including Merkel and French President Emmanuel Macron have voiced concern for Navalny, who has faced repeated physical attacks and prosecutions in more than a decade of opposition to Russian authorities.

The US embassy in Moscow said in a tweet on Friday that if the poisoning claim proved true it would represent “a grave moment for Russia, and the Russian people deserve to see all those involved held to account”.

Navalny lost consciousness shortly after his plane took off on Thursday from Tomsk in Siberia, where he was working to support opposition candidates ahead of regional elections next month.

Yarmysh said he had seemed “absolutely fine” before boarding the flight and had only consumed a cup of tea at the airport.

She said she was sure he had suffered from an “intentional poisoning” and put the blame on Putin.

“Whether or not he gave the order personally, the blame lies with him,” she said.

Kremlin spokesman Dmitry Peskov said claims of poisoning were “only assumptions” unless tests proved otherwise.

He wished Navalny a “speedy recovery” after pledging Kremlin help to secure him treatment abroad if needed.

Navalny has made many enemies with his anti-corruption investigations, which often reveal the lavish lifestyles of Russia’s elite and attract millions of views online.

He has suffered physical attacks in the past, including a 2017 incident where he endured chemical burns to his eye after green dye was splashed on his face.

Last year Navalny said he suspected poisoning when he suffered rashes and his face became swollen while serving a short jail term after calling for illegal protests.

He has been the target of multiple criminal probes and spent numerous stretches in police cells for organising illegal protests, while his Anti-Corruption Foundation was regularly raided by police and investigators.

The latest incident follows several infamous poisonings of Kremlin critics in the past.

Britain named two Russian spies as suspects after Russian double agent Sergei Skripal and his daughter Yulia were poisoned with the nerve agent Novichok in the city of Salisbury in March 2018.

Former Russian security service agent Alexander Litvinenko was fatally poisoned with radioactive polonium in a cup of tea in London. Russia refused to extradite chief suspect Andrei Lugovoi, who became a nationalist MP after the 2006 attack.

Several other opposition figures have suffered severe illnesses in Russia that they blamed on poisoning.(AFP)