Alexei Navalny's aides have said that they believe he was poisoned with a cup of tea.
The German government said Monday it is "fairly likely" that Russian opposition figure Alexei Navalny, who is currently being treated in a Berlin hospital, was poisoned.
The 44-year-old was brought to Berlin on Saturday from Siberia, where he fell ill on a flight with what Russian doctors have blamed on a metabolic disorder.
"We are dealing with a patient who it is fairly likely was poisoned," Angela Merkel's spokesman Steffen Seibert told reporters.
The Kremlin critic, Russia's best-known opposition figure, was rushed into intensive care on Thursday after his plane made an emergency landing in the Siberian city of Omsk.
Aides have said they believe Navalny was poisoned with a cup of tea, pointing the blame at President Vladimir Putin.
The Omsk regional health ministry said Saturday that caffeine and alcohol were found in Navalny's urine, but "no convulsive or synthetic poisons were detected".
"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," Seibert said.
Navalny was flown to Berlin on a plane chartered by the German NGO Cinema for Peace, an initiative financed by private donations.
The transfer came after German Chancellor Angela Merkel extended an offer of treatment in Germany, saying news of Navalny's condition had "truly upset me".
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