Hurricane Irma on Thursday pounded more of the Caribbean, shredding homes and weather records and leaving at least 12 people dead as it honed in on the United States, where up to a million people were told to flee.

The evacuation of coastal areas of Florida and neighboring Georgia was the biggest seen in the US in a dozen years, as Brock Long, head of the US Federal Emergency Management Agency, warned: “It will be truly devastating.”

“The entire southeastern United States better wake up and pay attention.”

Barreling across the Caribbean, the rare Category Five Irma wielded monster winds and torrential rain, wreaking destruction on tiny islands like St Martin, where 60 per cent of homes were wrecked, before slamming into the US Virgin Islands and Puerto Rico.

In its westward rampage, Irma packed winds of up to 185 miles per hour (mph), an intensity that it sustained for 33 hours — the longest of any storm since satellite monitoring began in the 1970s.

The latest bulletin from the Miami-based National Hurricane Centre included the first hurricane and storm surge warnings for south Florida and the Florida Keys, even as winds slowed slightly to 165 mph.

The storm left devastation in its wake. The International Red Cross said 1.2 million people had already been hit by Irma, a number that could rise to 26 million.

Tossed aside like matchsticks

On many islands, roofs were ripped off buildings as if by a giant's hand, shipping containers were tossed aside like matchsticks and debris flung far and wide, and airports, sea ports and mobile phone networks were knocked out.

At least two people were killed in Puerto Rico, a senior rescue official said. More than half of the territory's population of three million was without power, with rivers breaking their banks in the center and north of the island.

Governor Ricardo Rossello activated the National Guard and opened storm shelters sufficient for up to 62,000 people.

Authorities on the US Virgin Islands also reported four deaths.

“We lost a significant and a good number of assets... in terms of fire stations, police stations,” Governor Kenneth Mapp said in a Facebook post, adding that the region's main health facility, the Schneider Regional Medical Center, lost its roof.

Lisa Poser, a spokeswoman for Mapp, told AFP the territory's critically wounded were transported to Puerto Rico and warned there may be more casualties as authorities continue their search and rescue.

St Martin, a pristine island resort divided between France and the Netherlands, also suffered the full fury of the storm.

France said four had died and 50 were injured, two of them seriously. Sixty percent of homes were so damaged that they were uninhabitable.

The Netherlands said the storm killed at least one person and injured several others on the Dutch part of St Martin.

The Netherlands said it was racing to provide food and water for 40,000 people over the next five days, while France said more than 100,000 packages of combat rations were en route.

Britain said it was sending two warships to help victims in the Caribbean, and earmarking $41 million in aid.

'Everything is destroyed'

Irma also laid waste to tiny Barbuda which suffered “absolute devastation,” with up to 30 per cent of properties demolished, Prime Minister Gaston Browne said.

Rescue teams were scouring the island for survivors, an AFP correspondent said, as some 300 people without shelter were evacuated to Antigua.

One person is known to have died on the island of 1,600 residents, apparently a child whose family was trying to get to safer ground.

As of Thursday night the eye of the monster storm was located a few dozen miles northeast of Great Inagua island in the Bahamas.

Poor Haitians were left to face Irma's fury alone as authorities showed little sign of preparing for what forecasters said could be a catastrophic event.

Two people were injured when an uprooted coconut tree crashed onto their home near Cap-Haitien, authorities said, adding that the situation was dire in northeastern Ouanaminthe after the adjoining Massacre River overflowed, cutting off food supplies from the neighboring Dominican Republic.

Cuba moved 10,000 foreign tourists from beach resorts in the exposed part of the island, and hiked its disaster alert level to maximum.

Florida is expecting to face the brunt of the storm from Friday night, with forecasters warning of sea-level surges of up to 25 feet above normal tide levels.

Tourists in the popular Key West islands were packing their bags on a mandatory evacuation order, with a similar order for residents due to follow.

“We can't save you once the storm starts,” Governor Rick Scott told a press conference.

James Nickolos, a 69-year-old theology professor in Miami, said: “This morning I went to the beach as I do every morning, and it was very empty and beautiful but I had the feeling of watching a great beauty walking on a gang-plank to their death.”

Trump said he was “very concerned” about Irma, but added: “Florida is as well prepared as you can be for something like this, now it's just a question of what happens.” Irma hit the Caribbean as two other storms, Jose in the Atlantic Ocean and Katia in the Gulf of Mexico, were upgraded to hurricane status.

Katia, a Category One storm, is expected to hit the coast of the Mexican state of Veracruz before Friday. Jose, following in the path of Irma and located east of the Lesser Antilles late Thursday, strengthened to a Category Three event, packing winds of up to 120 miles per hour, the US National Hurricane Center said.