More than a million people in the U.S. were told to evacuate Saturday as a strengthened Hurricane Dennis headed to the US Gulf coast, after killing at least 15 people in the Caribbean.

Dennis was upgraded to a category three hurricane with sustained winds of 185 kilometres an hour after an Air Force plane flew over the storm to gauge the strength of its winds.

"Additional strengthening is expected," the National Hurricane Centre warned.

The hurricane moved into the Gulf of Mexico, with its centre passing well off Key West, a city at the southern tip of a chain of islands linked to mainland Florida by a series of bridges.

But heavy rains and fierce winds on the edge of the hurricane battered the popular tourist destination as well as other parts of southern Florida.

Concern, however, focused on Alabama and western Florida where the hurricane was expected to make landfall on Sunday.

Both states are still recovering from the hurricane damage of 2004 - particularly Florida, which was hit by four storms in quick succession last year.