MOSCOW: 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.

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.