Hurricane Irma ripped through the Caribbean on Friday, leaving a trail of devastation and killing 17 as it barrelled towards the United States where up to a million people have been told to flee.

So far, 1.2 million people have been affected by Irma, the Red Cross said.

But that number looks set to rise -- and could reach as high as 26 million, the agency said.

With the monster storm expected to reach the American south by the weekend, coastal areas of Florida and Georgia were battening down the hatches and carrying out their biggest evacuation since 2005.

"The entire southeastern United States better wake up and pay attention," warned US federal emergency chief Brock Long. "It will be truly devastating."

Roaring across the Caribbean, the rare Category Five hurricane laid waste to a series of tiny islands like St Martin, where 60 percent of homes were wrecked, before slamming into the Virgin Islands and Puerto Rico.

By Friday morning, the US National Hurricane Center (NHC) had downgraded Irma to Category Four with maximum wind speeds of up to 155 miles per hour (250 kilometres per hour) while warning it was still extremely dangerous.

On many islands, violent winds have ripped roofs and facades off buildings, hurling lumps of concrete, cars and even shipping containers aside.

- Death and destruction -

At least two people were killed in Puerto Rico, and more than half of its three million residents were without power after rivers broke their banks in the center and north of the island.

Another four people were killed on the US Virgin Islands, with the governor's office saying a number of badly injured people had been airlifted to Puerto Rico.

One person died in tiny Barbuda which also suffered "absolute devastation," with 300 people evacuated to Antigua and up to 30 percent of properties demolished.

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

France said at least nine people had been killed across its Caribbean territories and seven more were missing. There were 112 people injured, including two seriously, said Interior Minister Gerard Collomb.

Six out of 10 homes were left uninhabitable, with insurers in Paris estimating their overall costs would likely be "much higher" than 200 million euros ($240 million).

On the Dutch side of St Martin, one person died, officials said. Dutch King Willem-Alexander will head to the island of Curacao to the south on Sunday for a briefing on the aid operation, and may travel on to St Martin, officials said.

"It's as if a bomb went off. Before everything was so beautiful and green here. Now everything is as grey as a Dutch winter," retired police officer Klaas Groen told Dutch newspaper Algemeen Dagblad.

A state of emergency has been declared in the British Virgin Islands where residents spoke of scenes of "total devastation".

"Our downstairs doors suddenly blew out, which was terrifying," Emily Killhoury told the BBC from her home in Tortola.

"We eventually emerged at about 7:00 pm to see total devastation."

Britain's defence ministry said it was sending two military transport planes to the region carrying personnel, supplies and recovery equipment.

- 'Looting' -

As European nations quickly mobilized to help their citizens in the Caribbean, French and Dutch ministers said they were sending hundreds of extra police to St Martin tackle a spate of "looting".

The storm has caused major shortages of food, water and petrol.

"The situation is serious," Dutch Prime Minister Mark Rutte said Friday when asked about reports of looting on the island.

Speaking to Algemeen Dagblad, one witness reported seeing "people with guns and machetes" in the street.

French Overseas Territories Minister Annick Girardin said 400 police officers would be deployed after seeing "pillaging right in front of us" in St Martin where most of the 80,000 inhabitants have lost their homes.

In the Dominican Republic, torrential rain and powerful winds whipped the northern and eastern coasts, leaving 17 districts cut off.

Nearly 20,000 people were evacuated and more than 100 houses destroyed.

And in Cuba, some 10,000 foreign tourists were evacuated from beach resorts as authorities hiked the disaster alert level to maximum.

- 'Walking the plank' -

As of early Friday, the eye of the storm was just north of Great Inagua, at the southern tip of the Bahamas.

The worst of the storm is expected to hit Florida Keys and the southern Florida Peninsula on Sunday, the NHC said, with forecasters warning of storm surges of up to 25 feet (almost eight meters) above normal tide levels.

After going for a walk along the beach in Miami, theology professor James Nickolos, 69, spoke of a sense of impending doom.

"I had the feeling of watching a great beauty walking on a gang-plank to their death," he said.

Trump said he was "very concerned" about the situation but insisted Florida was "well prepared" for the storm.

As Irma cut through the Caribbean, two other storms in the region were upgraded to hurricane status: Jose in the Atlantic Ocean and Katia in the Gulf of Mexico.

Katia is expected to hit the coast of the Mexico on Friday, while Jose -- a Category Three storm -- is following Irma's path, the NHC said.

Still reeling from Irma, many islanders were bracing for the next storm, with Jose bearing down hard.

"I hear we could take another direct hit," Carlos Suarez Menendez told Dutch television RTL from Philipsburg on St Martin.

"That will be terrible."