Hurricane Irma began pummeling Florida late Saturday, threatening almost the entire southeastern US state after cutting a deadly path of destruction through the Caribbean.

Tens of thousands of Floridians were hunkering down in shelters for a direct hit from the monster storm, after more than 6.3 million -- nearly a third of the state's population -- were ordered to evacuate.

For those still at home, it was already too late to escape the wrath of what could be the worst hurricane in storm-prone Florida.

"If you have been ordered to evacuate anywhere in the state, you need to leave right now. Not tonight. Not in an hour. Now. You are running out of time to make a decision," Governor Rick Scott said hours before wind gusts began to lash the island chain known as the Florida Keys.

He said some 76,000 people had already lost power, cautioning that "it's going to get worse... This is going to be massive."

Mass exodus

In Florida, cities on both the east and west coasts of the peninsular state took on the appearance of ghost towns, as nervous residents heeded insistent evacuation orders.

The storm was expected to move along or near Florida's southwest Gulf coast by Sunday afternoon.

But Irma is so wide that authorities were bracing for destructive storm surges on both coasts and the Keys, the chain of low-lying islands that stretch south of Miami toward Cuba.

And hurricane-force winds are expected to lash the peninsula as it rolls north toward Georgia.

A tornado funnel cloud has already formed off the coast of Fort Lauderdale, with the NHC warning that "a few" more were possible in south and central Florida.

On highway 75 along the western coast of Florida, a steady stream of cars pressed northward as thousands fled at the last minute from the fast-approaching killer hurricane.

Bumper-to-bumper traffic snaked north out of the state, with mattresses, gas cans and kayaks strapped to car roofs.

Strip malls, fast food restaurants and retail giants were all closed for business.

In Key West, police opened a "shelter of last resort" for those who had ignored mandatory evacuation orders.

Scott Abraham, who lives on the fifth floor of a beachfront apartment building in Miami Beach, is planning to ignore evacuation orders and ride the storm out with his wife and two kids.

"If I lived in a house I would have left, but if it gets flooded here it's going to take a week at least to come back. I don't want that," he said.

Warning that Irma would be worse than Hurricane Andrew -- which killed 65 people in 1992 -- Scott, Florida's governor, said all 20.6 million Floridians should prepare to flee.

Cuban-American Orlando Reyes, 82, was forced to leave his assisted living facility in Miami Beach.

"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."

Path of destruction

The storm smashed through a string of Caribbean islands, beginning with tiny Barbuda on Wednesday, followed by the holiday islands of Saint Barthelemy and Saint Martin.

Also affected were the US Virgin Islands, Puerto Rico, the Dominican Republic, Haiti and the Turks and Caicos. The Bahamas were spared Irma's worst.

"Houses are smashed, the airport is out of action," Saint Barthelemy resident Olivier Toussaint said.

"Upside-down cars are in the cemeteries. Boats are sunk in the marina, shops are destroyed."

Another powerful storm, Category Four Hurricane Jose, was heading toward the same string of Caribbean islands Irma has pummeled in recent days, though it was now forecast to be weaker than initially expected.

The deteriorating weather grounded aircraft and prevented boats from bringing relief supplies to hard-hit islands.

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