German doctors treating Russian opposition leader Alexei Navalny for a suspected poisoning said Friday the dissident is still in an induced coma but his condition is stable and his symptoms are improving.

Navalny, a politician and corruption investigator who is one of Russian President Vladimir Putin’s fiercest critics, fell ill on a flight back to Moscow from Siberia about a week ago and was taken to a hospital in the Siberian city of Omsk after the plane made an emergency landing.

Last weekend, he was transferred to the Charité hospital in Berlin, where doctors found indications of “cholinesterase inhibitors” in his system.