Hurricane Irma has begun shifting northward, putting the low-lying Florida Keys directly in its path as tens of thousands of Floridians crowded into shelters and hundreds of thousands more were added to mandatory evacuation orders.
The weather began deteriorating in the Florida Keys late on Saturday and in other parts of southern Florida, the National Hurricane Centre said in its 5pm advisory. Major hurricane force winds were expected over the Keys by sunrise on Sunday, the centre said.
A mandatory evacuation, ordered for the Keys days ago, meant most people have already left. The head of the Federal Emergency Management Agency said anyone who stayed on the archipelago will have to wait for help if they need it.
"There is no safe area within the Keys, and you put your life in your own hands by not evacuating," Brock Long told CNN News.
Residents of the Keys were among 6.3 million people ordered to evacuate the state, including 700,000 added by Governor Rick Scott on Saturday.
Traffic was heavy on roads leading out of Florida and emergency shelters started filling up after officials warned of "major life-threatening impacts from coast to coast" across the peninsula, which is home to 20 million people.
The state's emergency management agency said nearly 70,000 people were hunkering down in more than 420 shelters opened in the state.
The National Hurricane Center said Irma remained a category 3 hurricane, with 205km/h winds at its centre, but added that it is forecast to regain strength after moving away from Cuba.
The hurricane centre predicts Irma will follow a course along the western side of Florida - which includes the major cities of Fort Meyers and Tampa - on Sunday. People who didn't evacuate were told to brace for the storm by pulling in lawn furniture, wind chimes and anything else unsecured on their property.
US President Donald Trump said his administration is monitoring the situation around the clock and is "as prepared as we can be". Speaking from the presidential retreat at Camp David, Maryland, he urged people to heed evacuation orders.
"All of America continues, I must say, to pray for families affected by Hurricane Harvey, Hurricane Irma, which looks like it's going to be a really bad one," he said, referring to an earlier hurricane that struck Texas in August.
Mr Trump said all of America grieves for the lives already lost.
Irma's eye was about 195km southeast of the Florida Keys on Saturday evening moving at about 15km/h, the hurricane centre said, warning that storm surges of as much as three metres were possible.
Officials imposed a night-time curfew in Miami Beach on the eastern side of the state, which also is expected to feel the wrath of the massive storm whose hurricane-force winds extend more than 100km from its centre.
Irma, one of the strongest storms ever to impact the region, caused major destruction earlier this week when it struck the US and British Virgin Islands, Anguilla, Saint Martin, Saint Barthelemy, and St Kitts and Nevis after developing into a Category 5 storm.
At least five people have died in British overseas territories, according to media reports on Saturday, including one on the island of Anguilla and another four on the British Virgin Islands.
Media reports previously said up to 24 could be dead as a result of the storm, with French Interior Minister Gerard Collomb saying the storm left at least nine dead on the French side of Saint Martin and St Barthelemy.