Hurricane Irma began pummeling Florida late on 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.

More than 170,000 homes and businesses in Florida have lost power and the centre of Irma is about 140km southeast of Key West.

Florida Power & Light said on its website that more than half of those outages were in the Miami-Dade area, where about 600,000 people have been ordered to evacuate.

"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.

At North Collier Regional Park, a designated shelter just outside the city of Naples, anxious evacuees prayed they and their loved ones would remain safe when the storm made landfall.

"All we wanted to make sure is to feel safe and whatever happens we just have to start I guess from the beginning," Viviana Sierra said.

MacDill Air Force Base, the military installation home to US Central Command, issued mandatory evacuation orders with the eye of the storm expected to pass over its home city of Tampa early Monday. The Kennedy Space Center was also closed.

The White House said President Donald Trump, Vice President Mike Pence and their cabinet were briefed on Hurricanes Irma and Jose, with Trump warning on Twitter that "this is a storm of enormous destructive power".

After blasting through the nearby Cuban coastline, Irma weakened from a maximum-strength Category 5 to a Category 3 storm, but then strengthened again to a Category 4, with 210km/h winds, as it approached south Florida.

With near-hurricane force winds lashing the Florida Keys starting around 8:00pm (01:00GMT), the Miami-based National Hurricane Center (NHC) warned that "Irma is forecast to re-strengthen" as it approaches mainland Florida.

There was a serious threat of flooding from storm surges of up to 4.5 metres along Florida's west coast - enough to cover a house.

At least 25 people have been killed since Irma began its devastating march through the Caribbean earlier this week.

Terrified Cubans who rode out Irma in coastal towns after the storm made landfall on Friday on the Camaguey archipelago reported "deafening" winds, uprooted trees and power lines, and blown rooftops.

There were no immediate reports of casualties, but officials reported "significant damage". A total of 1.5 million people were evacuated.

"When Irma eventually hits land it might be a Category 3, which is significantly less than what was predicted," said Al Jazeera's Alan Fisher, reporting from Miami.

"But that is still a major storm and still could cause significant destruction across Florida."

While Miami would likely "escape the worst", Key West "is going to be hammered", Fisher said.

"This was a city that was prepared for the worst and has been preparing since Monday to what might come their way," Fisher said.

Michael Hernandez, adviser to Miami's mayor, told Al Jazeera: "We've taken unprecedented measures to protect our residents.

"We feel that we have done all that we can ... whether this storm hits us as a Category 5, 4, or what they're saying since it's moved away - a Category 1 or 2. It doesn't matter. We made the right call."

Mass exodus

In Florida, cities on both the east and west coasts 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 towards Cuba.

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And hurricane-force winds are expected to lash the peninsula as it rolls north towards 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 Interstate 75, a major highway 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, petrol 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.

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 4 Hurricane Jose, was heading towards 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 mobilising 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.