Brock Long, the administrator of Federal Emergency Management Agency (FEMA), has said that Hurricane Irma will “devastate” either Florida or neighboring states.
He said parts of Florida would be without power for days. Half a million people in the state have been ordered to leave their homes.
Hurricane Irma has left a trail of destruction in the Caribbean, affecting an estimated 1.2 million people.
At least 20 people are known to have died so far.
Irma has been downgraded to a category 4 storm, but officials warn that it remains “extremely dangerous”.
According to the US National Weather Service, Irma was expected to bring wind speeds of around 165mph over the weekend as it hits Florida.
Brock Long said: “Hurricane Irma continues to be a threat that is going to devastate the United States in either Florida or some of the south-eastern states.”
“The entire south-eastern United States better wake up and pay attention,” he added.
The death toll continued to rise on September 8 in the Caribbean.
France’s Interior Minister Gérard Collomb said nine people were dead and seven missing in the French territory on St Martin, an island shared with the Netherlands, and St Barthélemy, known more commonly as St Barts. Another death – the second – has been confirmed in the Dutch territory of Sint Maarten.
French officials said six out of 10 homes on Saint-Martin were so badly damaged that they were uninhabitable.
The US Consulate General in Curacao said it believes an estimated 6,000 Americans are stranded on the island.
French, British and Dutch military authorities have deployed aid – including warships and planes equipped with food, water and troops – to their territories.
Irma, which most recently lashed the Turks and Caicos Islands and brought torrential rain to the Dominican Republic and Haiti, was headed towards Cuba and the Bahamas.
The worst of the storm is expected to hit east and central Cuba, with the eye of the storm predicted to pass between the north coast of Cuba and the Bahamas, the National Hurricane Center (NHC) said.
About 50,000 tourists are fleeing or have fled Cuba, with resorts on the north coast now empty, Reuters reports.
A huge evacuation of south-eastern, low-lying coastal areas in the Bahamas has been ordered. The tourism ministry said in a video statement that thousands of tourists left before the storm’s arrival.
Meanwhile Brock Long predicted a “truly devastating” impact on Florida.
South Florida “may be uninhabitable for weeks or months” because of the storm, the US National Weather Service said.
On the archipelago of Turks and Caicos, with its population of about 35,000, one witness described a drop in pressure that could be felt in people’s chests.
Irma ripped off roofs on the capital island, Grand Turk, flooded streets, snapped utility poles and caused a widespread black-out.
Irma also caused some damage to roofs, flooding and power outages in the northern parts of the Dominican Republic and Haiti.
Irma is due to hit Florida as a category 4 hurricane on September 10, bringing storm surges and flooding.
Florida Governor Rick Scott said on September 7: “If you look at the size of this storm, it’s huge.”
“It’s wider than our entire state and could cause major and life-threatening impacts on both coasts – coast to coast.”
President Donald Trump said: “I can say this: Florida is as well prepared as you can be for something like this. Now it’s just a question of what happens.”
Hurricane and storm surge warnings have been issued for south Florida and the Florida Keys, the NHC says.
President Trump’s own Mar-a-Lago estate in Palm Beach is among those ordered to evacuate, the Washington Post tweeted.
After ripping through Florida’s Atlantic coast, Irma is expected to move into Georgia and South Carolina.
Georgia Governor Nathan Deal said a mandatory evacuation on the state’s Atlantic coast was scheduled to begin on September 9.
Roads and airports have been jammed as thousands of people tried to evacuate areas at risk, with reports of fuel shortages and gridlock on some roads.
Many tourists are stranded with no seats left on flights back to their countries as flights to and from airports in Florida are being suspended.
Orlando’s international airport said commercial flights would stop from 17:00 local time on September 9.
Another storm, Jose, further out in the Atlantic behind Irma, is now a category 4 hurricane, with winds of up to 120mph.
Hurricane Katia, in the Gulf of Mexico, has strengthened to a category 2 storm, with winds of up to 85mph. A warning is in effect for the coast of the Mexican state of Veracruz and the storm is expected to make landfall on September 9.