"This is a serious concern ... We need to wait and see how this situation develops and also get confirmation of what happened," he said.
O'Brien also wished Navalny a speedy recovery and stressed that the United States would "closely monitor" the situation.
Earlier, German Chancellor Angela Merkel and French leader Emmanuel Macron commented on Navalny's hospitalization.
The Ministry of Health of the Omsk Region reported that Navalny's condition is regarded as consistently grave.
On Friday, August 21, specialists from the N.M. N.I. Pirogov and Moscow Research Center of Neurosurgery named after Academician N.N. Burdenko to hold an extended consultation.
On August 20, the plane on which Navalny flew from Tomsk to Moscow urgently landed in Omsk due to the fact that he became ill. He was hospitalized.