Store owners boarded up their windows and families sandbagged their homes to join a mass exodus on Friday as Hurricane Irma churned towards Florida after cutting a deadly swath through the Caribbean.

Warning that Irma would be worse than Hurricane Andrew, which killed 65 people in 1992, Florida's governor said all of the state's 20.6 million inhabitants should be prepared to evacuate.

Bumper-to-bumper traffic snaked north out of the peninsula, with mattresses, gas cans and kayaks strapped to car roofs as residents heeded increasingly insistent warnings to get out.

"THIS IS AS REAL AS IT GETS," screamed a warning posted by the National Weather Service for Key West -- one of the low-lying islands off South Florida that will be the first to face Irma's fury on Saturday night.

"NOWHERE IN THE FLORIDA KEYS WILL BE SAFE. YOU STILL HAVE TIME TO EVACUATE."

North of the Keys, in Miami Beach, Orlando Reyes, an 82-year-old Cuban-American, had to flee his assisted living facility at a moment's notice.

"It is frightening," he told AFP at a shelter in Miami. "We had to leave without a cent, without taking a bath, or bringing anything along."

President Donald Trump warned residents in Irma's path they were facing a threat of "epic proportion, perhaps bigger than we have ever seen.

"Be safe and get out of its way, if possible," he tweeted.

Roaring across the Caribbean, the monster storm claimed at least 17 lives as it laid waste to a series of tiny islands like Saint Barthelemy and Saint Martin, where 60 percent of homes were wrecked and looting has broken out, before slamming into the Virgin Islands and Puerto Rico.

"Houses are smashed, the airport is out of action, telephone and electricity poles are on the ground," Olivier Toussaint, a resident of Saint Barthelemy, told AFP. "Upside-down cars are in the cemeteries. Boats are sunk in the marina, shops are destroyed."

Irma was downgraded overnight from a rare Category Five storm to a still-deadly Category Four, and continued to pack extremely dangerous winds of 150 miles per hour (240 kilometers per hour).

Even as Irma barrelled towards Florida, meteorologists were closely monitoring two other hurricanes: Jose -- a Category Four storm following Irma's path in the Atlantic -- and Katia, set to reach Mexico on Friday.

Caribbean relief disrupted

Hurricane Jose was playing havoc with emergency operations in the Caribbean as the deteriorating weather prevented boats from leaving with relief supplies and grounded aircraft.

Irma's powerful winds on Friday began whipping the east and center of Cuba, where close to a million people have left their homes to stay with relatives or in official shelters.

The Caribbean's biggest island, Cuba has already evacuated 10,000 foreign tourists from beach resorts and raised its disaster alert level to maximum as Irma drew near.

Havana is expected to be spared the worst, but remains on hurricane alert.

In Florida, where forecasters warned of storm surges of up to 25 feet (almost eight meters), more than half a million people are facing mandatory evacuation orders -- triggering a mass exodus complicated by traffic gridlock and fuel shortages.

Normally bustling Miami Beach was deserted and storefronts were boarded up with plywood, some bearing graffiti reading "Say no to Irma" or "You don't scare us Irma."

Hurricane Irma has laid waste to a string of islands in the Caribbean and is now heading northwest towards Florida. Photo: AFP

"Nobody can be prepared for a storm surge. They can destroy everything," said David Wallack, a 67-year-old salsa club owner doing his best to secure his property on the city's Ocean Drive.

"We just can pray for the best. You put what you can in a suitcase and hope."

Police cars crawled the coastal roads of Florida's West Palm Beach, blaring out "Attention, attention, this is a mandatory evacuation zone, please evacuate."

In neighboring Georgia, Governor Nathan Deal also ordered a mandatory evacuation of the city of Savannah, population 150,000, and other coastal areas.

According to the Miami-based National Hurricane Center, Irma is expected to strike the Florida Keys late on Saturday before moving inland.

'Powerful and deadly'

At 5:00pm (2200 GMT) on Friday, the hurricane was over the northern edge of Cuba and the central Bahamas and heading west at around 12 miles per hour (19 kph).

"The storm is powerful and deadly," warned Florida Governor Rick Scott.

"Do not ignore evacuation orders," Scott said. "All Floridians should be prepared to evacuate soon."

"Remember, we can rebuild your home, we can't rebuild your life."

The US military was mobilizing thousands of troops and deploying several large ships Friday to aid with evacuations and humanitarian relief, as the Air Force removed scores of planes from the southern United States.

In the Caribbean, violent winds ripped roofs and facades off buildings, hurling lumps of concrete, cars and even shipping containers aside.

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 a number of badly injured people airlifted to Puerto Rico.

One person died in tiny Barbuda where 30 percent of properties were demolished and 300 people had been evacuated to Antigua.

France said at least nine had been killed across its Caribbean territories with seven more missing. There were 112 people injured, two seriously.

On the Dutch side of St Martin, one person died, officials said.

In northwest Haiti, a motorcyclist was missing after trying to cross a flooded river and a number of roads were washed out.

European nations quickly mobilized to help their citizens in the Caribbean, with France and the Netherlands ordering hundreds of police to St Martin to tackle an outbreak of looting amid major shortages of food, water and petrol.

The French government said 400 police officers would be deployed following an outbreak "pillaging" in St Martin where most of the 80,000 inhabitants have lost their homes.