Hurricane Ian walloped Florida, pounding the southern US state's coast with extreme wind and rain, and causing "catastrophic" flooding from destructive storm surges.

The National Hurricane Center (NHC) said the eye of the "extremely dangerous" hurricane made landfall just after 3pm local time on the barrier island of Cayo Costa, west of the city of Fort Myers.

Dramatic TV footage from the coastal city of Naples showed floodwaters surging into beachfront homes, submerging roads and sweeping away vehicles.

The NHC said Ian was packing maximum sustained winds of 240 kilometers per hour when it made landfall and forecast "catastrophic storm surge, winds and flooding in the Florida peninsula."

Ian, an "extremely dangerous" Category 4 storm, was destined to affect several million people across Florida and in southeastern states Georgia and South Carolina.

More than one million customers have already lost power in Florida, a tracking website recorded, with the number expected to rise.

Of 11 million customers tracked in Florida, 1.07 million were suffering outages, PowerOutages.us reported.

As hurricane conditions spread, forecasters warned of a looming once-in-a-generation calamity.

A meteorology student measures wind gusts as the hurricane approaches Sarasota, Florida

The NHC said Ian was bringing sustained winds of 250 kilometres per hour, just shy of Category 5 intensity - the strongest on the Saffir-Simpson scale.

Some 2.5 million people were under mandatory evacuation orders in a dozen coastal Florida counties, with several dozen shelters set up, and voluntary evacuation recommended in others.

For those who decided to ride out the storm, authorities were stressing it was too late to flee and that residents should hunker down and stay indoors.

US police said a boat carrying migrants sank and left 23 people missing and four survivors as Hurricane Ian slammed into the Florida coast today.

"US Border Patrol agents... responded to a migrant landing in Stock Island, Florida," Miami chief patrol agent Walter Slosar said on Twitter.

"Four Cuban migrants swam to shore after their vessel sank due to inclement weather."

Airports in Tampa and Orlando stopped all commercial flights, and some quarter million households were already without power.

But that was a "drop in the bucket" compared with the outages expected over the next 48 hours, Florida Governor Ron DeSantis said.

"This is going to be a nasty, nasty day, two days," he added.

"Clearly this is a very powerful major hurricane that's going to have major impacts."

With conditions rapidly deteriorating, some thrill-seekers nevertheless were seen walking in the mud flats of Tampa Bay and further south at Port Charlotte's Charlotte Harbor, ahead of Ian's arrival.

With up to two feet of rain expected to fall on parts of the so-called Sunshine State, and a storm surge that could reach devastating levels of 12 to 18 feet above ground, authorities were warning of dire emergency conditions.

"This is a life-threatening situation," the NHC warned.

The storm was set to move across central Florida before emerging in the Atlantic Ocean by late Thursday.

Wind and rain pick up in the Ybor City neighborhood ahead of Hurricane Ian

Widespread blackout

Ian a day earlier had plunged all of Cuba into darkness after battering the country's west as a Category 3 for more than five hours before moving back out over the Gulf of Mexico.

The storm damaged Cuba's power network and left the island "without electrical service," state electricity company Union Electrica said.

Only the few people with gasoline-powered generators had electricity on the island of more than 11 million people.

Others had to make do with flashlights or candles at home, and lit their way with cell phones as they walked the streets.

"Desolation and destruction. These are terrifying hours. Nothing is left here," a 70-year-old resident of the western city of Pinar del Rio was quoted as saying in a social media post by his journalist son, Lazaro Manuel Alonso.

At least two people died in Pinar del Rio province, Cuban state media reported.

People walk through a flooded street in Batabano, Cuba

'Historic event'

In the United States, the Pentagon said 3,200 national guardsmen had been called up in Florida, with another 1,800 on the way.

FEMA (Federal Emergency Management Agency) administrator Deanne Criswell warned that Ian's "painful impacts" were being felt even before the hurricane's landfall.

National Weather Service director Ken Graham echoed concerns about what lies ahead, expressing certainty Ian will leave a trail of destruction.

"This is going to be a storm we talk about for many years to come," he said. "It's a historic event."

As climate change warms the ocean's surface, the number of powerful tropical storms, or cyclones, with stronger winds and more precipitation is likely to increase.

The total number of cyclones, however, may not.

According to Gary Lackmann, a professor of atmospheric science at North Carolina State University, studies have also detected a "potential link" between climate change and what is known as rapid intensification - when a relatively weak tropical storm surges to a Category 3 hurricane or higher in a 24-hour period, as happened with Ian.

"There remains a consensus that there will be fewer storms, but that the strongest will get stronger," Lackmann told AFP.

Ms Criswell said one of the top concerns was the safety of Florida's large elderly population.

Many have health and mobility issues or are in hospitals, nursing homes and other facilities that are difficult to evacuate.

President Joe Biden, speaking at an event in Washington, vowed that the federal government will help Florida after the storm passes.

"We are on alert and in action, we've approved every request Florida has made," Mr Biden said.