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 (CNN) — The German government says it is “fairly likely” that Russian opposition leader Alexey Navalny was poisoned and will therefore need special protection.

Navalny is being treated in a Berlin hospital after falling ill on a flight from Siberia last week. He was transferred to the German capital from the Siberian city of Omsk on Saturday morning.

“We are dealing with a patient who, it is fairly likely, was poisoned,” Chancellor Angela Merkel’s spokesman, Steffen Seibert, told journalists during a press briefing on Monday. “Because there is a certain probability of a poison attack, protection is necessary,” Seibert said.

Navalny’s spokesperson Kira Yarmysh said last week that he fell sick from suspected poisoning on a flight to Moscow from the Siberian city of Tomsk.

Navalny, a critic of Russian President Vladimir Putin, has been receiving treatment at Berlin’s Charite Hospital which, according to Seibert, will be providing updates on the opposition leader’s condition. Jaka Bizilj, chairman for Cinema for Peace Foundation, which organized the medical evacuation, told CNN on Saturday that Navalny was in a “stable condition.”

The Siberian hospital that had previously been treating Navalny on Friday rejected claims he had been poisoned — even as his wife said the doctors there could not be trusted.

On Friday, Anatoly Kalinichenko, the deputy chief physician at the Russian hospital where Navalny was being treated told a news conference that no poisons were found in Navalny’s blood or urine. “We don’t believe that the patient suffered poisoning,” Kalinichenko told local journalists.

“Poisons or traces of their presence in the body have not been identified. Probably, the diagnosis of ‘poisoning’ remains somewhere in the back of our minds. But we do not believe that the patient suffered poisoning,” he added.