US personnel have reported brain damage and other symptoms of the mysterious "Havana Syndrome."
CIA Director Bill Burns warned Russian intel services over the cases, The Washington Post reported.
The warning was hedged, suggesting the US doesn't know if Russia is involved, the report said.
The CIA director warned Russia of "consequences" if the country turns out to be behind the "Havana Syndrome" illnesses afflicting American personnel around the world, The Washington Post reported, citing US officials.
Bill Burns gave the warning confidentially to the heads of Russia's security and intelligence agencies while he was in Moscow earlier this month, The Post reported, citing the unnamed officials.
The Post noted that Burns tempered his warning by saying that the consquences would come "if" Russia was found to be involved in US personnel falling ill.
The Post noted that the US did not blame Russia, and that the way the warning was delivered suggests the US does not yet know the cause of the illnesse or whether Russia had a role at all.
More than 200 Americans across the world have reported a mysterious illness with symptoms that include nausea, headaches, vertigo, and memory loss. Some have reported brain damage.
The cause of it remains unknown, and theories include a microwave weapon.
It was nicknamed "Havana Syndrome" as it was first reported among US personnel in Havana, Cuba, in 2016.
The US is working to try and understand its source.
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