Anticipating that Hurricane Irma will "devastate" part of the United States, officials are preparing a massive response to the storm, the head of the Federal Emergency Management Agency says.
With Irma set to hit Florida as early as Saturday night, parts of Florida were expected to lose electricity for days, if not longer, and more than 100,000 people may need shelter, FEMA administrator Brock Long warned at a news conference.
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"Hurricane Irma continues to be a threat that is going to devastate the United States in either Florida or some of the southeastern states," Mr Long said.
Mr Long warned people not to ignore evacuation orders.
"They need to get out and listen and heed the warnings," Mr Long said.
US President Donald Trump urged residents in its path to heed government recommendations.
"This is a storm of absolutely historic destructive potential. I ask everyone in the storm's path to be vigilant and to heed all recommendations from government officials and law enforcement," Mr Trump said in a videotaped statement.
Irma was a Category 5 hurricane, the most dangerous measure by the National Hurricane Center, before being downgraded to Category 4 early on Friday (local time) after pummelling islands in the Caribbean.
Officials have thousands of personnel ready to respond and millions of meals and litres of water in place nearby, Mr Long said.
The National Weather Service said that Friday was the last day to evacuate before winds would start to reach unsafe speeds in Florida.
Airlines added extra flights from Florida on Thursday before announcing plans to halt service from some southern Florida airports starting Friday afternoon.
US Health and Human Services Secretary Tom Price called Irma a "remarkably dangerous storm and the window to get yourself in the right spot... is closing rapidly".
Mr Price said the main hospital in St Thomas in the US Virgin Islands was closed after being damaged by Irma, and critically ill patients were being evacuated to Puerto Rico or other islands.
On Friday, the US House of Representatives voted 316-90 to approve a measure to more than double funding to US$15.25 billion (NZ$21 billion) for FEMA and local block grants to handle natural disasters after the Senate passed the measure on Thursday, 80-17.
FEMA's disaster assistance fund could have run out of money on Friday without action. President Donald Trump is expected to sign the measure on Friday.