MARIGOT, France (AFP) — Hurricane Irma on Thursday pounded the Caribbean on its likely collision course for the United States, shredding homes, prompting thousands to flee and killing at least six people.

Up to a million people were ordered to leave coastal areas of Florida and neighbouring Georgia — the biggest mass evacuation in the US in a dozen years.

“It will be truly devastating,” warned the head of the US Federal Emergency Management Agency (FEMA), Brock Long.

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

Barrelling 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.

“It looks as if an enormous lawnmower descended from the sky and drove over the island,” witness Marilou Rohan told NOS public broadcaster.

“Houses have been flattened. People are helpless, you can see it in their eyes,” Rohan added, saying desperate survivors were looting supermarkets.

In its westward rampage, Irma packed winds of up to 295 kilometres (183 miles) per hour, 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 put the winds at 175 mph as the storm headed for the Bahamas.

Devastation was left in the storm’s wake. The international Red Cross said 1.2 million people had already been hit by Irma, a number that could rise to 26 million.

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.

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

France said four had died and 50 were injured, two of them seriously. Sixty per cent 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, where communications were all but cut off.

French Prime Minister Edouard Philippe described the disaster as “unimaginable and unprecedented.”

“The work will be long, emotions will run deep and the sadness will be great,” he said.

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. A 200-member French team flew in to Guadeloupe to coordinate rescue efforts, headed by Overseas Territories Minister Annick Girardin.

Britain said it was sending two warships to help victims in the Caribbean, and earmarking £32 million (US$41 million) in aid. The first vessel was expected to reach affected territories on Thursday.

Speaking to Dutch broadcaster RTL, Koen, a 20-year-old who lives in the town of Voorhout of St Martin, said he was shocked by what he saw.

“There is huge damage. Sand has been blown over everything. Everything is destroyed.”

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

“Barbuda now is literally rubble,” he said.

“These storms are more ferocious, they are coming in greater frequency — evidence that climate change is real,” Browne added in an interview with CNN.

More than half of Puerto Rico’s population of three million was without power, with rivers breaking their banks in the centre and north of the island where Governor Ricardo Rossello activated the National Guard and opened storm shelters sufficient for up to 62,000 people.

Also in Irma’s path was the British archipelago of the Turks and Caicos, where some islands were being evacuated.

“We have started to fill our shelters because a number of people who live in very low-lying areas are very vulnerable,” the territory’s governor, John Freeman, told the BBC.

“We’ve also engaged in terms of getting people off the island who are here, North American tourists and others.”

Irma was expected to hit the northern edges of the Dominican Republic and Haiti later Thursday, continuing past eastern Cuba before veering north towards Florida.

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

US President Donald Trump has already declared a state of emergency for Puerto Rico, the US Virgin Islands and Florida, and the southern US state of Georgia ordered the mandatory evacuation of the city of Savannah and other coastal areas.