But unlike with Harvey, dangerous winds will barely abate once Irma makes landfall on Sunday morning.

Hurricane Irma turned its fury toward the Florida Keys on Saturday after setting off one of the largest evacuations of Americans from a storm and completing a destructive march along Cuba's northern coast.

Irma was expected to rip through Florida's southern archipelago on Sunday morning as a Category 4 storm, the second-highest designation on the Saffir-Simpson scale. Wind gusts near hurricane force began to batter the Florida Keys late on Saturday, the National Hurricane Centre said.

The storm's enormity over the past several days has daunted even veteran forecasters. Hurricane force winds extended 112 km from Irma's center as it veered toward Florida, a state around 240 km wide.

Irma, which killed at least 22 people in the Caribbean, was considered a life-threatening danger to Florida as well and could inflict a natural disaster causing billions of dollars in damage to the third-most-populous U.S. state.

Tracking models showed Irma would make landfall on the western side of the Florida peninsula and heading up the coast, bringing 209 kph winds, storm surges up to 15 feet and flooding in some areas.

Amid urgent warnings from state officials to evacuate before it was too late, downtown Miami was all but abandoned on Saturday. Sheets of rain swept through the deserted city of 400,000 people, forming large puddles in empty streets that are usually filled with tour buses and taxi cabs.

The wind sent a construction crane spinning on the roof of the Miami Worldcenter, a billion dollar mixed-use project near the home of the Miami Heat basketball team and the city's performing arts center.

On Florida's west coast, resident Charley Ball said he expected a storm surge to completely engulf the island of Sanibel where he lives.

“Just left the island and said goodbye to everything I own," said Ball, 62.

Hurricane experts rattled

Irma, located about 168 km southeast of Key West on Saturday night, was a Category 5 storm, the highest ranking possible, when it crashed into Cuba during the morning.

It gradually weakened to a Category 3 storm as it bumped along the island's northern coastline, flooding streets and sending waves crashing over sea walls.

Maximum sustained winds stayed around 201 kph, the NHC said. Irma is expected to regain strength as it steams over warm waters south of Florida.

Irma will dump up to 25 cm to 51 cm of rain over Florida and southeast Georgia through Monday, the National Weather Service said, a fraction of what Hurricane Harvey dropped on Texas and Louisiana two weeks ago, killing 60 people and causing an estimated $180 billion in property damage.

But unlike with Harvey, dangerous winds will barely abate once Irma makes landfall on Sunday morning.

Tom Durr, a 66-year-old retired physician who along with his wife, Lorraine, fled their house near a large bay on the Gulf Coast on Tuesday for a small farm they own in North Carolina, said he does not expect much to be left when they return.

It will be a nice waterfront lot in sunny Florida with no house, no trees, no cars, no boats and an amazing view of devastation as far as you can see.

The Durrs house is in Englewood, between Fort Myers and Sarasota near where the eye of Hurricane Irma is forecast to hit land.

Destruction in Cuba

In Cuba, the destruction along the north central coast was similar to that suffered by other Caribbean islands over the last week as Irma plowed into Ciego de Avila province.

Residents venture out after the passing of Hurricane Irma in Caibarien, Cuba on Saturday. | Photo Credit: AP

Cubans walked through ankle-deep water in Caibarien, a fishing town where streets were flooded and covered in seaweed. Elsewhere, winds toppled trees and utility polls or ripped apart roofs.

“This was the strongest storm Caibarien ever had. It will take a while to recover from this, at least a few years.” said Risle Echemendia, 28.

It was the first time the eye of a Category 5 storm had made landfall in Cuba since 1932, state media said, and the island's Communist government ordered the evacuation of more than a million people from its path.

Officials in Florida have ordered a total of 6.3 million people, or about a third of the state's population, to evacuate, creating massive traffic jams on highways and overcrowding shelters.

In Palm Beach, President Donald Trump's waterfront Mar-a-Lago estate was under evacuation order.

“This is a storm of enormous destructive power, and I ask everyone in the storms path to heed ALL instructions from government officials,” Mr. Trump said on Twitter.

A total of some 9 million people in Florida may lose power, some for weeks, the Florida Power & Light Co said.

The window for people in evacuation zones to flee was drawing to a close on Saturday, officials said, warning that gas stations would soon be without fuel and bridges would be closed in some areas. But the storm's unpredictability made even evacuation challenging.

Chris Cardona and his wife Laurie left their mobile home near Miami on Thursday to seek refuge with friends near Tampa.

“Not only did we go west, but so did Irma. She's tracking us, that feisty minx,” Cardona, 54, said by phone.

Irma could cause insurance losses of between $15 billion and $50 billion in the United States, catastrophe modeling firm AIR Worldwide said.