Russian opposition leader Alexei Navalny, who Germany says was poisoned by a weapons-grade Novichok nerve agent, is now out of a medically induced coma and is being weaned off mechanical ventilation, the Berlin hospital treating him said Monday.

"He is responding to verbal stimuli," Charite hospital said in a statement, reporting that the 44-year-old's condition "has improved".

However, the hospital said it was too early to determine the long-term impact of the poisoning.

Navalny fell ill after boarding a plane in Siberia last month. He was initially treated in a local hospital before being flown to Berlin for treatment.

Russian opposition leader Alexei Navalny fell ill on a flight from Siberia on Thursday
Russian opposition leader Alexei Navalny fell ill on a flight from Siberia on Thursday AFP / Yuri KADOBNOV

The German government said last week that toxicology tests have found "unequivocal evidence" that Novichok was used on Navalny, prompting Chancellor Angela Merkel and her Western counterparts to demand answers from Russia.

Earlier Monday, Merkel's spokesman underlined that she would not rule out consequences on the multi-billion-euro Nord Stream 2 pipeline project if Moscow failed to thoroughly investigate the case.

The Kremlin has however denounced attempts to blame the poisoning on Russia as "absurd".

Novichok is a military-grade poison that was developed by the Soviet government towards the end of the Cold War and can be deployed in an ultra-fine powder, liquid or vapour.

It was used against ex-double agent Sergei Skripal in Britain in 2018, a poisoning that the West believes was ordered by the Kremlin.