BERLIN (AFP) - The Berlin hospital treating leading Kremlin critic Alexei Navalny said on Friday (Aug 28) there was "some improvement" in his condition after he suffered a suspected poisoning.

The 44-year-old nevertheless remains in a medically induced coma and on a ventilator, Berlin's renowned Charite hospital said in a statement, describing his condition as serious but not life-threatening.

"There has been some improvement in the symptoms" caused by the poisoning, the statement said.

"However, due to the severity of the patient's poisoning, it remains too early to gauge potential long-term effects."

Navalny, a leading opposition figure, fell ill after boarding a plane in Siberia last week.

He was initially treated in a local hospital before being flown to Berlin for treatment last Saturday.

Charite doctors said they believed the anti-corruption campaigner was poisoned with a substance that inhibits the cholinesterase enzyme, a feature of nerve agents.

Navalny's allies say he may have been poisoned by a cup of tea he drank at Tomsk airport in Siberia.

But the Russian doctors who first treated Navalny said their tests did not find any toxic substances, and the Kremlin has rejected international calls for an investigation.

Navalny's spokeswoman Kira Yarmysh confirmed in a tweet on Friday that she had received an update on Navalny's health from German doctors.

She said Navalny's condition was "serious" and that the doctors were refraining "from giving any prognosis".