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".

Navalny, a 44-year-old anti-corruption campaigner and one of President Vladimir Putin's fiercest critics, fell ill on a domestic flight last month and was treated in a Siberian hospital before being evacuated to Berlin.

Germany said last week there was "unequivocal evidence" that Navalny had been poisoned with Novichok, the same substance used in the 2018 attack on a former Russian double agent and his daughter in the English city of Salisbury.

Navalny's associates say the use of Novichok, a military-grade nerve agent, shows that only the Russian state could be responsible, but the Kremlin fiercely denies any involvement.