Hurricane Matthew has strengthened as it nears a "direct hit" with Florida, and is set to be the strongest storm in the United States for 12 years.
The hurricane has pounded the Bahamas after devastating parts of Haiti.
Damage could be "catastrophic", Florida's governor said. More than two million people have been evacuated.
Matthew, with winds of 125mph (205km/h), is expected to hit southern Florida later on Thursday as a Category Four storm, officials say.
At least 108 people have been killed in Haiti and thousands displaced. The storm has forced the presidential election there to be postponed.
Four people were also killed in the neighbouring Dominican Republic.
As of 13:10 GMT, the storm was passing over the capital of the Bahamas, Nassau. All air and sea traffic has been halted and people urged to move to higher ground because of storm surges.
The latest from Florida
Hurricane Matthew has been upgraded into a Category Four storm, the second-highest classification.
It is expected "move very close" to the east coast of the Florida peninsula on Thursday night, the US National Hurricane Center (NHC) said.
States of emergency have already been declared in four southern states, allowing the National Guard to be deployed. On Thursday afternoon, President Obama declared a state of emergency for Florida, allowing federal emergency teams to be deployed.
Matthew could be the first Category Four storm to make landfall in the United States since Charley in August 2004, that caused $14bn (£11bn) in southern states.
"There are no excuses, you need to leave," Florida Governor Rick Scott warned everyone in evacuation zones.
"If you're reluctant to evacuate, just think of all the people this storm has already killed. You and your family could be among these numbers if you don't take this seriously."