The German hospital treating leading Russian opposition politician Alexey Navalny has said tests indicate that he was poisoned.
Berlin's Charite hospital said in a statement on Monday the team of doctors who have been examining the 44-year-old since he was admitted on Saturday have found the presence of "cholinesterase inhibitors" in his system.
They said at the moment the specific substance is not yet known.
The hospital said "the patient is in an intensive care unit and is still in an induced coma. His health is serious but there is currently no acute danger to his life".
German Chancellor Angela Merkel called on Russia to fully investigate the Kremlin critic's suspected poisoning saying those responsible "must be identified and held accountable".
"In view of the prominent role of Mr Navalny in the political opposition in Russia, the authorities there are now urgently called upon to investigate this act down to the last detail - and in full transparency," she said in a joint statement with Foreign Minister Heiko Maas.
The two also expressed hope Navalny would make a full recovery and extended their best wishes to his family.
Police posted at hospital
Navalny was brought to the German capital from Siberia, where he fell ill on a flight with what Russian doctors have blamed on a metabolic disorder.
He was rushed into intensive care on Thursday after his plane made an emergency landing in the Siberian city of Omsk.
Aides have said they believed poison was administered to Russia's best-known opposition figure in a cup of tea, pointing the blame at President Vladimir Putin.
But Russian health officials said on Monday that Navalny had tested negative for cholinesterase inhibitors when he was hospitalised in Omsk last week, contradicting a diagnosis made by German doctors treating him in Berlin.
Navalny showed no signs of having been poisoned when he was admitted to the clinic and tests were carried out on him to check for a wide range of substances, including inhibitors, the health ministry in Omsk said in a statement.
The Omsk regional health ministry said on Saturday caffeine and alcohol were found in Navalny's urine, but "no convulsive or synthetic poisons were detected".
Merkel's spokesman Steffen Seibert earlier on Monday told reporters: "The suspicion is ... that somebody poisoned Mr Navalny - that somebody seriously poisoned Mr Navalny - which, unfortunately, there are some examples of in recent Russian history, so the world takes this suspicion very seriously."
He also said Berlin police and federal agents were posted at the hospital.
"It was obvious that after his arrival, protective precautions had to be taken," he said.
