Berlin (CNN) German doctors say there is a "high plausibility" that a member of the Russian protest group Pussy Riot, who was taken ill in Russia last week, was poisoned.

Speaking at a press conference Tuesday, Dr. Kai-Uwe Eckardt of the Berlin Charite Hospital said that an external substance appears to have affected Pyotr Verzilov's nervous system.

Doctors have been unable to determine the nature of the substance or the source, he said, adding that while the activist remains in intensive care, his life is no longer in danger.

"The information we currently have... shows a high plausibility that poisoning has taken place here," Eckardt said. "To turn it around, so far we have no indication that there might be another explanation for his state."

The announcement adds weight to claims made by other Pussy Riot members on Thursday that Verzilov was poisoned in Russia. The Russian punk band -- known for obscuring their identities with colorful balaclava-style masks -- is an outspoken critic of the Putin government.

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