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

Navalny, a corruption investigator who is one of Russian President Vladimir Putins 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. Found in some drugs, pesticides and chemical nerve agents, cholinesterase inhibitors block the breakdown of a key chemical in the body, acetycholine, that transmits signals between nerve cells.

He is being treated with the antidote atropine. Charité said Friday that there has been some improvement in the symptoms caused by the inhibition of cholinesterase activity.

While his condition remains serious, there is no immediate danger to his life, the hospital said. However, due to the severity of the patients poisoning, it remains too early to gauge potential long-term effects.