Hurricane Milton has marched across Florida, whipping up deadly tornadoes, destroying homes and knocking out power to more than three million customers.

However, the Tampa Bay metropolitan area appears to have escaped without the catastrophic flooding that had been feared.

Authorities were still waiting for rivers to crest but so far water levels were at or below what they received with Hurricane Helene two weeks ago, Tampa Mayor Jane Castor said on Thursday morning.

"So we'll have storm surge damage but nothing like it could have been. It could have been catastrophic for Tampa Bay," she told MSNBC on Thursday.

The storm hit Florida's west coast on Wednesday night as a category-three hurricane on the five-step Saffir-Simpson scale, with top sustained winds of 205km/h.

While still a dangerous storm, this was less violent than the rare category-five hurricane that had threatened the state as it trekked over the Gulf of Mexico towards Florida.

At least two deaths were reported at a retirement community following a suspected tornado in Fort Pierce on the east coast.

Milton weakened as it crossed land, dropping to a category-one hurricane with top sustained winds of 145km/h as it reached the peninsula's east coast, the National Hurricane Center said.

By Thursday morning, the storm was moving away from the Florida Atlantic coast, lashing communities on the eastern shoreline.

(Hurricane Milton's winds shredded the roof of Tropicana Field stadium in St Petersburg, Florida. (AP PHOTO)

The eye of the storm had hit land in Siesta Key, a barrier island town of some 5,400 people off Sarasota about 100km south of Tampa Bay.

Florida Governor Ron DeSantis said he hoped the Tampa Bay area, home to 3 million people and once seen as the potential bull's eye, could dodge major damage. The hurricane still left its mark, tearing open the roof of Tropicana Field, the stadium of the Tampa Bay Rays baseball team.

The winds also toppled a large construction crane in St Petersburg, sending the structure crashing down onto a deserted street.

Milton also spawned at least 19 tornadoes, the governor said, causing damage in numerous counties and destroying about 125 homes, most of them mobile homes.

In Fort Myers on the southwest coast, resident Connor Ferin surveyed the wreckage of his home, which had lost its roof and was full of debris and rainwater after a tornado suddenly hit.

"All this happened instantaneous, like these windows blew out," he said.

Floridians face a new clean-up from Hurricane Milton after being battered by Helene two weeks ago. (EPA PHOTO)

"I grabbed the two dogs and run under my bed and that was it. Probably one minute total."

St Lucie County's Pearson estimated 100 homes were destroyed in the county where some 17 tornadoes touched down, NBC said.

More than three million homes and businesses in Florida were without power, according to PowerOutage.us.

In a state already battered by Hurricane Helene two weeks ago, as many as two million people had been ordered to evacuate before Milton's arrival, and millions more live in the path of the storm.

Much of the southern US experienced the deadly force of Helene as it ripped through Florida and several other states.

Both storms are expected to cause billions of dollars in damage.

Hurricane Milton hit the Florida coast as a category-three storm with winds of 195km/h. (EPA PHOTO)

As of Thursday morning, 2209 US flights had been cancelled, according to flight tracking website FlightAware, with the highest number cancellations from Orlando, Tampa and southwest Florida.

While human evacuees jammed the highways and created petrol shortages, animals including African elephants, Caribbean flamingos and pygmy hippos were riding out the storm at Tampa's zoo.

Nearly a quarter of Florida's petrol stations were out of fuel on Wednesday afternoon.

The Federal Emergency Management Agency had moved millions of litres of water, millions of meals and other supplies and personnel into the area.

None of the additional aid would detract from recovery efforts for Hurricane Helene, agency administrator Deanne Criswell said.

Trucks had been running 24 hours a day to clear debris left behind by Helene before Milton potentially turned them into dangerous projectiles, DeSantis said.

About 9000 National Guard personnel were deployed in Florida, ready to assist recovery efforts, as were 50,000 electricity grid workers in anticipation of widespread power outages, he said.

By Brad Brooks and Leonara LaPeter Anton in ST. PETERSBURG