Hurricane Irma gave Florida a coast-to-coast pummelling with winds up to 210 km/h Sunday, swamping homes and boats, knocking out power to millions and toppling massive construction cranes over the Miami skyline.

The 640-kilometre-wide storm blew ashore in the morning in the mostly cleared-out Florida Keys, then began a slow march up the state's west coast, its winds extending clear across to Miami on the Atlantic side.

Irma was nearing the heavily populated Tampa-St. Petersburg area late Sunday, though in a much-weakened state. While it arrived in Florida a Category 4 hurricane, by nightfall it was down to a Category 2 with winds of 160 km/h. Meanwhile, more than 160,000 people waited in shelters statewide as Irma headed up the coast.

There were no immediate reports of deaths in Florida. In the Caribbean, at least 24 were people were killed during Irma's destructive trek. 

Bryan Koon, Florida's emergency management director, said late Sunday that authorities had only scattered information about the storm's toll, but he remained hopeful.

"I've not heard of catastrophic damage. It doesn't mean it doesn't exist. It means it hasn't gotten to us yet," Koon said.

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In the low-lying Keys, where a storm surge of over three metres was recorded, appliances and furniture were seen floating away, and Monroe County spokeswoman Cammy Clark said the ocean waters were filled with navigation hazards, including sunken boats. But the full extent of Irma's wrath there was not clear.

The county administrator, Roman Gastesi, said crews would begin house-to-house searches Monday to check on survivors. And an airborne relief mission, led by C-130 military cargo planes, was gearing up to bring emergency supplies to the Keys.

Storm surge was a big concern. The National Hurricane Center said a federal tide gauge in Naples reported  more than two-metre rise in water levels in just 90 minutes late Sunday.

Many streets were flooded in downtown Miami and other cities.

In downtown Miami, two of the two dozen construction cranes looming over the skyline collapsed in the wind. A third crane was reported down in Fort Lauderdale. No injuries were reported.

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Water rises up to a sidewalk by the Miami River as Hurricane Irma arrives in south Florida on Sunday. (Carlos Barria/Reuters)

Damage, flooding and curfews

An apparent tornado spun off by Irma destroyed six mobile homes in Palm Bay, midway up the Atlantic coast. Flooding was reported along Interstate 4, which cuts across Florida's midsection.

Fort Lauderdale police arrested nine people they said were caught on TV cameras looting sneakers and other items from a sporting goods store and a pawn shop during the hurricane.

More than 3.3 million homes and businesses across the state lost power, and utility officials said it will take weeks to restore electricity to everyone.

About 30,000 people heeded orders to leave the Keys as the storm closed in, but an untold number refused, in part because to many storm-hardened residents, staying behind in the face of danger is a point of pride.

John Huston, who stayed in his Key Largo home, watched his yard flood even before the arrival of high tide.

Cuba Hurricane Irma

Strong waves brought by Hurricane Irma hitting the Malecon seawall in Havana can be seen late Saturday. (Ramon Espinosa/Associated Press)

"Small boats floating down the street next to furniture and refrigerators. Very noisy," he said by text message. "Shingles are coming off."

Statewide storm

Irma made landfall just after 9 a.m. at Cudjoe Key, about 32 kilometres outside Key West.

Meteorologist Ryan Maue of WeatherBell Analytics said the entire Florida peninsula will be raked by Irma's right front quadrant — the part of a hurricane that usually brings the strongest winds, storm surge, rain and tornadoes.

Some 643 kilometres north of the Keys, people in the Tampa-St. Petersburg area started bracing for the onslaught. The Tampa Bay area, with a population of about three million, has not taken a direct hit from a major hurricane since 1921.

Tampa Bay is vulnerable in a powerful storm because the bay acts as a funnel for storm surges, forcing water into narrow channels with nowhere else to go. 

"I've been here with other storms, other hurricanes. But this one scares me," Sally Carlson said as she snapped photos of the waves crashing against boats in St. Petersburg.

After leaving Florida, a weakened Irma is expected to push into Georgia, Alabama, Mississippi, Tennessee and beyond. A tropical storm warning was issued for the first time ever in Atlanta, some 350 km from the sea.

Disaster declaration

U.S. President Donald Trump approved a disaster declaration for Florida, opening the way for federal aid.

"Once this system passes through, it's going to be a race to save lives and sustain lives," Federal Emergency Management Agency chief Brock Long said on Fox News Sunday.

Florida's governor activated all 7,000 members of the Florida National Guard, and 10,000 guardsmen from elsewhere were being deployed.

Irma heads to Fort Myers, Fla.1:53

Irma at one time was the most powerful hurricane ever recorded in the open Atlantic, a Category 5 with a peak wind speed of 300 km/h.

For days, forecasters had warned that Irma was taking dead aim at the Miami area and the rest of the state's Atlantic coast.

But then Irma made a more pronounced westward shift that put a bull's-eye on the Tampa area — the result of what meteorologists said was an atmospheric tug-of-war between weather systems that nudged Irma and determined when it made its crucial right turn into Florida.