Aides to Alexei Navalny said they believed he had been poisoned because of his political activities.
The Kremlin said Thursday it was aware that President Vladimir Putin's chief critic Alexei Navalny was ill and wished him well, but said there was no evidence yet to back claims he had been poisoned.
"We are reading this information... We know that he is in serious condition... Like with any citizen of the Russian Federation we wish him a speedy recovery," Kremlin spokesman Dmitry Peskov told journalists.
Aides to Navalny, who was in intensive care in a hospital in the Siberian city of Omsk, said they believed he had been poisoned because of his political activities.
Mr Peskov said there was no evidence yet of any poisoning.
"As far as we know there are not yet any test results. All assumptions are only assumptions," he said. "Whether it was a poisoning needs to be confirmed by a laboratory."
If tests confirmed there was a poisoning, he said, "then there will be an investigation" just like with any Russian citizen.
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