Hurricane Irma will "devastate" part of the US, authorities warned, as one of the most powerful Atlantic storms in a century closed in on the state of Florida after lashing the Caribbean with devastating winds and rain and killing at least 18 people.
Irma was about 724 km southeast of Miami, Florida, early on Friday (local time) after saturating the northern coasts of the Dominican Republic and Haiti and pummelling the Turks and Caicos Islands, ABC News reported.
The "extremely dangerous" hurricane was downgraded from a category five to a category four early on Friday but still packed winds as strong as 240 km per hour, the National Hurricane Centre (NHC) said in an advisory.
Head of the US' Federal Emergency Management Agency (FEMA) Brock Long said US officials were preparing a massive response to the storm.
"Hurricane Irma continues to be a threat that is going to devastate the United States in either Florida or some of the southeastern states," he said.
Long warned that parts of Florida would be out of electricity for days, if not longer, and that more than 100,000 people may need shelter.
The National Weather Service said Friday was the last day to evacuate before winds would start to hit unsafe speeds in Florida.
Airlines said on Thursday they were adding extra flights from Florida before announcing plans to halt service from some southern Florida airports starting on Friday afternoon.
Health and Human Services Secretary Tom Price said Irma was a "remarkably dangerous storm, and the window to get yourself in the right spot ... is closing rapidly".
A massive evacuation has clogged Florida's major highways as residents tried to escape, but traffic officials decided against reversing the direction of southbound lanes because they still needed to move petrol and supplies south.
The toll from the storm has risen as emergency services got access to remote areas pummelled by heavy winds and rain.
Irma led to at least 18 deaths in the Caribbean, including nine in French territories, one at the British overseas territory of Anguilla, one on Barbuda, four on the US Virgin Islands, and three in Puerto Rico.
Meanwhile, Cuban authorities also evacuated 700,000 people from the country's northeastern coast ahead of the hurricane hitting the island nation, authorities said.