The Russian opposition leader Alexei Navalny remains in an induced coma at a Berlin hospital after a suspected poisoning but doctors say his condition is stable and his symptoms are improving.

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

He was transferred at the weekend 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.

Navalny is being treated with atropine, the same antidote used after the 2018 nerve agent attack in Salisbury on the former Russian spy Sergei Skripal and his daughter.

Charité said on Friday “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 patient’s poisoning, it remains too early to gauge potential long-term effects.”

Navalny’s allies say he was deliberately poisoned and that the Kremlin was responsible. These accusations have been rejected by Russian officials as “empty noise”.

Navalny was brought to Germany for treatment after the chancellor, Angela Merkel, personally intervened.

“We have an obligation to do everything so that this can be cleared up,” Merkel told reporters at her annual summer news conference on Friday. “It was right and good that Germany said we were prepared to take in Mr Navalny. And now we will try to get this cleared up with the possibilities we have, which are indeed limited.”

Merkel said that when there was more clarity about what happened, Germany would try to ensure there was a response from Europe. She cited the poisoning of the Skripals, which prompted many European countries to expel Russian diplomats.

Navalny’s team submitted a request last week for Russia to launch a criminal investigation into a possible attempted murder, but said there had been no response.

The Kremlin spokesman Dmitry Peskov said he saw no grounds for a criminal inquiry until the cause of the politician’s condition was fully established. Russian prosecutors said on Thursday that a preliminary inquiry had not found any indication of deliberate criminal acts committed against Navalny.