Cuba watchers from Miami to Washington were on high alert Tuesday, reacting to widely circulating but vague reports that Fidel Castro's health had taken a serious turn for the worse.

While recognizing that this kind of speculation happens periodically -- most recently in August 2007 -- those who deal with Washington on a regular basis say authorities aren't taking it lightly.

''High sources in Washington are saying that reliable sources have said that he has taken gravely ill,'' said University of Miami's Andy Gomez, who serves as an advisor to the U.S. Task Force on Cuba, an arm of the Brookings Institution think tank comprised of academics and former diplomats. ``They are monitoring this very closely, including looking for additional movements of security and troops. So far, none of this has happened.''

Castro's continued absence from public view, an unusually long break from published essays, failure to schedule private chats with recent visiting presidents and veiled remarks by Venezuelan ally Hugo Chávez has elevated the unconfirmed reports of the Cuban leader's pending death.

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