Russia defied threats of new sanctions over the poisoning of Alexei Navalny, as US President Donald Trump said on Friday he had not yet seen proof that the Kremlin critic was a victim of Moscow's Novichok programme.
A new crisis in ties between Russia and the West broke out after Germany said this week there was "unequivocal evidence" that President Vladimir Putin's top foe had been poisoned using the Soviet-era nerve agent.
Western leaders and many Russians have expressed horror at what Navalny's allies say is the first known use of chemical weapons against a high-profile opposition leader on Russian soil.
The 44-year-old lawyer fell ill on a Siberian flight last month and was evacuated to Germany for treatment. He has been in an artificially induced coma for the past two weeks.
The Kremlin again denied responsibility for the attack on Friday.
"A whole number of theories including poisoning were considered from the very first days," Putin's spokesman Dmitry Peskov told reporters. "According to our doctors, this theory has not been proved."
Foreign Minister Sergei Lavrov said Germany's justice ministry had so far failed to share any findings with Moscow's prosecutors, adding that Russian authorities had "nothing to hide".
Trump said Friday he had yet to see proof from Berlin that Navalny had been poisoned.
"I don't know exactly what happened. I think it's tragic, it's terrible, it shouldn't happen," the president told a press conference.
- 'Stress and dieting' -
Over the past few days, pro-Kremlin figures have wheeled out a number of eyebrow-raising theories, including that Navalny might have been poisoned by Germans in Berlin or even have poisoned himself.
On Friday, a toxicologist claimed the opposition politician's health could have deteriorated due to dieting, stress or fatigue, insisting no poison had been found in his samples in the Siberian city of Omsk.
"Any external factors could have triggered a sudden deterioration. Even a simple lack of breakfast," said the chief toxicologist for the Omsk region, Alexander Sabayev.
On Thursday, Belarusian strongman Alexander Lukashenko also muddied the waters, claiming his security forces had intercepted German calls showing Navalny's poisoning had been faked.
Russia has in the past denied responsibility for a 2018 Novichok attack on former double agent Sergei Skripal and his daughter in England, as well as a litany of similar incidents.
- NATO demands transparency -
In Brussels, NATO called for an international probe into Navalny's poisoning and demanded Moscow reveal details of its Novichok nerve agent programme to the OPCW global chemical weapons watchdog.
After an emergency meeting of NATO's ruling council, alliance chief Jens Stoltenberg said all members were united in condemning the "horrific" attack on Navalny.
Germany briefed the other 29 nations on the case and Stoltenberg said there was "proof beyond doubt" Novichok was used.
"The Russian government must fully cooperate with the Organisation for the Prohibition of Chemical Weapons on an impartial international investigation," Stoltenberg said.
The Novichok attack on Skripal in 2018 led to seven Kremlin diplomats being expelled from their NATO mission.
While Stoltenberg did not rule out a similar reprisal this time, he stressed that the Navalny poisoning was quite different from the Skripal attack, which happened on the soil of a NATO member.
European Union foreign policy chief Josep Borrell earlier called on Moscow to cooperate with an international probe into the poisoning and said the 27-nation bloc would not rule out sanctions.
In Moscow, a court threw out a complaint by Navalny's Anti-Corruption Foundation over investigators' perceived inaction, the group said.
Ivan Zhdanov, head of Navalny's anti-corruption group, said Thursday the opposition politician's poisoning opened a "new chapter" in a Kremlin crackdown on dissent.
"Now the Russian state will be inventing the most absurd and crazy versions of what has happened," he told AFP.