A bipartisan group of senators is urging Secretary of State Antony Blinken to take immediate action in response to proliferating reports of "Havana Syndrome," calling cases of the mysterious neurological condition that has sickened hundreds of U.S. officers "a significant, unmitigated threat to our national security," in a letter sent Wednesday. 

The letter, a copy of which was obtained exclusively by CBS News, follows dozens of recent, new reports of suspected cases in Colombia, Austria, Germany, Vietnam and other countries. In several instances, the incidents have occurred with startling proximity to senior...