At least 10 people have died in "nuclear" Hurricane Irma as the Category 5 storm continues to rip through the Caribbean towards Florida.

The hurricane devastated a string of Caribbean islands before it roared past the Dominican Republic towards Haiti. It's now headed towards Cuba and the US state of Florida. Miami is expected to take a direct hit.

Authorities in the US Virgin Islands say three people died in St Thomas and St Johns after Irma caused what they described as "catastrophic" damage.

The aftermath of Hurricane Irma in St. Martin. Irma cut a path of devastation across the northern Caribbean, leaving thousands homeless. (Photo / AP)

Irma also killed four people and injured about 50 on the French side of St Martin, an island split between Dutch and French control. A 2-year-old child was one of three people who died on the British island of Anguilla, independent Barbuda and the Dutch side of St Maarten.

Thousands more have been left homeless as the strongest Atlantic Ocean hurricane ever destroyed trees and buildings, cutting off islands.

Hurricane Irma

The huge storm has gusts of up to 360km/h and has broken records after sustaining 297km/h winds for 37 hours - the longest any tropical cyclone around the world has maintained that intensity.

Colorado State University hurricane expert Phil Klotzbach said it broke the previous record, held by Typhoon Haiyan, which had similar top winds for 24 hours before it hit the Philippines and killed 6000 people in 2013.

Irma also has been the most intense Atlantic hurricane on record outside the Caribbean and Gulf of Mexico, measured by its barometric pressure of 914 millibars.

According to the US National Hurricane Centre (NHC), winds have decreased slightly to 280km/h and Irma is expected to be downgraded to a Category 4 by the time it hits Florida on Sunday. But it's still expected to pack a punch.

A mandatory evacuation order is in place for the Florida Keys and that includes law enforcement. Florida Governor Rick Scott on Thursday urged people to leave before Irma's arrival.

"Do not try to ride out this storm," he warned residents. "We can't save you once the storm hits."

Storm damage in the aftermath of Hurricane Irma, in St. Martin. (Photo / AP)

In the next few hours Irma's eye is expected to move between Hispaniola and the Turks and Caicos Islands, and will then travel between the north coast of Cuba and the Bahamas during the next day or two.

Hurricane conditions are expected to continue in the Dominican Republic and Haiti tonight and will spread to the Bahamas by early Friday (local time).

Irma will reach the north coast of Cuba by late Friday and Saturday. Cuban residents are bunkering down but many of the 51,000 tourists on the island have already been evacuated.

Cuban Tourism Minster Manuel Marrero said Canada had evacuated all of its tourists, who made up an estimated 60 per cent of Cuba's visitors, according to ABC.

Evacuation orders are also in place for US residents in the Florida Keys, and are expected to be issued for parts of South Carolina and Georgia.

The eye of Hurricane Irma, left, is just north of the island of Hispaniola, with Hurricane Jose, right, in the Atlantic Ocean. (Photo / AP)

The aftermath of Hurricane Irma, in St. Maarten. (Photo / AP)

The 'big one'

More than half a million people have been ordered to leave Florida amid fears Irma could be the "big one" that obliterates the southeast coast.

The Big One has become a local myth: a massive hurricane that would obliterate the densely populated area, which is home to six million people.

The storm has triggered panic in the region, which includes Miami, Fort Lauderdale and West Palm Beach, clustered along a narrow ribbon of coastline that has seen nearly double-digit population growth over the past five years.

A mandatory evacuation order covers the Florida Keys, which could experience wind and rain from Friday. More than 30,000 have already left.

"Law enforcement is being evacuated out of the Florida Keys. If something happens to you there, it is likely that no one will be able to help you," Florida congressman Carlos Curbelo told CNN.

Hurricanes are standard for those living in Florida. In 1928, a hurricane caused Lake Okeechobee to burst its banks, unleashing a 6m wall of water that killed about 2500 people.

The "Great Miami Hurricane" of 1926 killed 372 people when it came ashore directly over the city, carrying with it a 3m storm surge.

Northbound traffic on the turnpike near Sunrise Blvd. is backed up in the rain as motorist prepare for Hurricane Irma. (Photo / AP)

Irma is set to be the strongest hurricane to hit southern Florida since Andrew in August 1992, which caused widespread damage south of Miami.

The Category 5 storm killed 15 people and indirectly caused the deaths of 25 more in Miami-Dade County alone. It destroyed 63,000 homes and caused a staggering $32.9 billion in damage.

But the state hasn't been directly hit by a major hurricane since Wilma in 2005.

If a Category 5 storm were to hit Florida again it would be far more catastrophic, causing $125b damage, according to reinsurance firm Swiss Re.

Rapid development and fast growth of the region has also made the coast more vulnerable to damage than in years past, the New York Times reported.

Path of destruction

Shocking aerial photos show the destruction Irma has already left in its wake. Authorities are scrambling to access the remote islands, many of which remain isolated without power and proper communication.

Unicef Haiti's chief of communications Cornelia Walther said the storm is "far stronger" than Hurricane Matthew, which smashed the island last year.

"Listening to the rain drumming over my head I think of those families who live in makeshift homes that barely stand straight in normal weather. What will their lives look like in 48 hours?" she said from the island.

A few of the homes that remained intact in the aftermath of Hurricane Irma, in St. Maarten. (Photo / AP)

Billionaire entrepreneur Richard Branson said he and his staff rode out Hurricane Irma on his private Caribbean island without suffering injuries, but the area is heavily damaged.

The head of the Virgin Group owns small Necker island in the British Virgin Islands. He said in a blog entry Thursday that he and the staff who stayed with him in a concrete cellar on the island were safe and well.

Branson said the area surrounding his home is "completely and utterly devastated". He said entire houses have disappeared and "I have never seen anything like this hurricane".

Outside the cellar he said bathroom and bedroom doors and windows were blown out. He said he was communicating via a satellite phone, but all other communications were down.

'Life threatening'

The US Hurricane Centre said Irma will bring "life threatening" wind, rain and storm surge to the Turks and Caicos Islands and Bahamas over the next two days.

Barbuda has been described as "barely habitable": more than 90 per cent of dwellings have been completely destroyed, a child killed and 60 per cent of the population are homeless.

US Federal Emergency Management Agency (Fema) chief Brock Long told CNN the hurricane will be "truly devastating" when it hits the southern coast of America.

"The majority of people along the coast have never experienced a major hurricane like this," he said.

The aftermath of Hurricane Irma in St. Martin. (Photo / AP)

Miami Beach Mayor Philip Levine said anyone left in the area "must leave".

"This storm will envelop us whether it is off shore or on shore. It won't make a difference," Levine told CBS4.

"I'll do anything in my power to convince [people] this is a very serious storm. This is a nuclear hurricane. They should leave the beach, they must leave the beach," he said.

"This could easily be the most costly storm in US history, which is saying a lot considering what just happened two weeks ago," said Brian McNoldy, a hurricane researcher at the University of Miami, alluding to the damage caused by Hurricane Harvey.

International response

Emergency services and aid agencies have gone into overdrive to prepare for the storm and clean up afterwards.

The UK is sending a military vessel to assist those in islands hit as well as £12 million ($21m) in aid after the hurricane did "critical damage" to the UK territory of Anguilla.

Junior Foreign Minister Alan Duncan said the country is "pulling out all the stops to make sure that we can do our utmost to bring urgent assistance".

Foreign Minister Boris Johnson reaffirmed the UK's commitment to respond to the "catastrophic damage" caused by Irma.

"What we will be doing now is making an urgent assessment of the further needs of communities in the British Virgin Islands and Anguilla to see what more can be done in terms of financial and humanitarian assistance," he added.

Juan Negron in front of whats left of his damaged property, after the passage of Hurricane Irma, in the island of Culebra, Puerto Rico. (Photo / AP)

Dutch Prime Minister Mark Rutte said the storm did damage of "epic proportions" to Dutch territory and left "widescale destruction".

"There is no power, no gasoline, no running water. Houses are underwater, cars are floating through the streets, inhabitants are sitting in the dark, in ruined houses and are cut off from the outside world," he said. The Dutch military is also sending two aircraft to co-ordinate aid drops.

President Trump tweeted that the US has "great teams of talented and brave people already in place and ready to help".

Australians due to travel to the region have been advised to contact the Smart Traveller website.