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Hurricane Irma will "devastate" either Florida or neighbouring states, the head of the US federal emergency agency has said.
Brock Long 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 1.2m people.
At least 20 people are known to have died so far.
It has been downgraded to a category four storm, but officials warn that it remains "extremely dangerous".
The US National Weather Service says that Irma was expected to bring wind speeds of around 165mph (270km/h) over the weekend as it hits Florida.
"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," Mr Long said.
"The entire south-eastern United States better wake up and pay attention," he added.
Florida Governor Rick Scott said all Floridians should be prepared for possible evacuation, and issued a stark warning to those in threatened areas.
"We are running out of time. If you are in an evacuation zone, you need to go now," he told reporters.
"Remember, we can rebuild your home, we can't rebuild your life."
The death toll continued to rise on Friday 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.
Reporting from another badly damaged island, Barbuda, the BBC's Laura Bicker says the destruction there is worse than feared.
(BBC)
