The Kremlin on Wednesday rejected international calls for an investigation into the poisoning of opposition leader Alexei Navalny, saying medics had not identified any toxin.

The 44-year-old lawyer and anti-corruption campaigner remains in a medically-induced coma in a Berlin hospital, where doctors said tests have shown he was poisoned.

Kremlin spokesman Dmitry Peskov slammed "hurried" statements on the "high likelihood" that Navalny was poisoned.

"We categorically disagree with them at this stage," he told journalists.

Navalny fell ill after boarding a plane to Moscow in Siberia and his aides believe he was poisoned by a cup of tea at the airport. They place the ultimate blame on President Vladimir Putin.

Navalny's family fought to medically evacuate him to Germany, saying they did not trust Russian doctors.

The doctors who treated Navalny for two days in the Siberian city of Omsk said their tests did not find any toxic substances, calling his condition a "metabolic disorder".

"How can we talk about a poisoning, if there's no poisonous substance?" Peskov said.

German doctors say their tests show Navalny was poisoned
German doctors say their tests show Navalny was poisoned AFP / Yuri KADOBNOV

No investigation has been opened in Russia despite Navalny's aides asking investigators to probe an attempted assassination.

Western leaders have urged Russia to immediately launch a transparent investigation.

The Charite hospital in Berlin treating Navalny said Monday that tests indicated poisoning with a substance that inhibits the cholinesterase enzyme, a feature of nerve agents.

The German hospital said the exact substance was unknown.

NATO chief Jens Stoltenberg on Wednesday called for an investigation saying there was "no reason to doubt" the German doctors' conclusions.

British Prime Minister Boris Johnson also urged a "full, transparent investigation," echoing earlier calls from US Secretary of State Mike Pompeo, the French foreign ministry and German Chancellor Angela Merkel.

The Kremlin spokesman shrugged off any responsibility, saying it was up to law enforcement authorities to decide what was "appropriate."

Kremlin officials refuse to speak of Navalny by name
Kremlin officials refuse to speak of Navalny by name AFP / Yuri KADOBNOV

"In any case this is not the prerogative of the presidential administration," Peskov said.

Russian parliament speaker Vyacheslav Volodin has instead called for MPs to examine whether foreign states set up the incident to discredit Russia.

Amid the focus on Navalny's case, Peskov has still avoided referring to him by name, using phrases such as "the sick man."

The practice dates back to the early days of Navalny's activism when the Kremlin sought to ignore the opposition movement.

"This is a Russian citizen who is in a coma and we would like to work out what was the cause of this coma," Peskov said.

Earlier Wednesday a Kremlin-linked businessman vowed to ruin Navalny by forcing him and his associates to pay court-ordered damages of around $1.2 million.

Yevgeny Prigozhin said he wanted to "strip this group of unscrupulous people of their clothes and shoes."

Prigozhin has run catering for the Kremlin and is nicknamed "Putin's chef." Media reports have said he funds a private army called Wagner, which he denies.

Prigozhin said that "if comrade Navalny kicks the bucket, I personally don't intend to persecute him in this world."

If Navalny survives, he will face "the full severity of the Russian law," he added.

The case concerned a video investigation by the Anti-Corruption Foundation founded by Navalny into a company making school dinners.

The company has been linked to Prigozhin in media reports though he denies owning it.

Navalny in July formally closed the Anti-Corruption Foundation in a move to avoid paying its share of the damages.

The Foundation's director Ivan Zhdanov in a message to supporters on Wednesday suggested Navalny could have been poisoned over one of its investigations that gain millions of views online.

But "it's clear the sanction to poison Navalny could have only been given by Putin," he claimed.