Hurricane Irma will devastate part of the US and officials are preparing a massive response to the storm, the head of the US Federal Emergency Management Agency says.

With Irma set to hit Florida at the weekend, FEMA Administrator Brock Long warned at a news conference that parts of Florida would be out of electricity for days, if not longer, and more than 100,000 people might need shelter.

“Hurricane Irma continues to be a threat that is going to devastate the United States in either Florida or some of the southeastern states,” Long said on Friday.

Irma had been a category-five hurricane before being downgraded to category-four early on Friday after pummelling islands in the Caribbean.
The US had experienced only three category-five storms since 1851 and Irma was far larger than the last one to hit the US in 1994, Hurricane Andrew, Long said.

He warned people not to ignore evacuation orders.

Officials had thousands of personnel ready to respond and millions of meals and litres of water in place nearby, Long said.

The National Weather Service said Friday was the last day to leave before winds would start to hit unsafe speeds in Florida.

Florida prepares

As southern Florida falls under hurricane warnings, petrol shortages and gridlock have plagued thousands of people fleeing for high ground ahead of Irma.

More than 500,000 people have been ordered to leave as the category-five hurricane tracks towards the state and that volume has turned usually simple trips into tests of will.

Carmen Pardo and her six-year-old daughter, Valeria, drove around Miami for seven hours frantically searching for somewhere to fill their petrol tank. They found nothing.

She eventually booked the only flight she could find leaving the city, to Orlando, where she reserved two seats on a bus bound for Tallahassee on Friday.

Late on Thursday, the National Hurricane Center issued the first hurricane warnings for the Keys and parts of southern Florida, including some of the Miami metropolitan area.

People along the Atlantic coast anxiously watched as Irma battered the northern Caribbean, killing at least 11 people and leaving thousands homeless.

At least 31,000 people fled the Florida Keys, which could begin seeing wind and rain from Irma as early as Friday night, Governor Rick Scott said.

With winds that peaked at 300km/h, Irma has been the most powerful hurricane ever recorded in the Atlantic.

NASA secured Kennedy Space Center, closing its doors to all non-essential staff, and SpaceX launched an unmanned rocket for an experimental flight.

Georgia Governor Nathan Deal has ordered evacuations and authorised about 5000 National Guard members to help with response and recovery.

The last category-five storm to hit Florida was Andrew in 1992, which killed 65 people and inflicted $US26 billion in damage.

– With agencies