The enormous Hurricane Ian has made landfall in the United States with fears it could have a "catastrophic impact" as it smashes its way through populated areas of Florida.

Forecasters are concerned as new pictures of Ian have revealed that the eye of the storm is as much as five times larger than previous deadly hurricanes.

There are warnings people "will not survive" if they are caught by five-metre high waves and winds of around 250km/h.

"This is going to be a storm we talk about for many years to come. It's a historic event," National Weather Service director Ken Graham said.

While no deaths have yet been reported on the US mainland, 23 Cuban migrants are reportedly missing after the boat their boat in sank off the Florida Keys on Wednesday. At least two people died when the storm went through Cuba on Tuesday.

The category 4 strength storm made landfall at Cayo Costa, just north of the city of Fort Myers, on Wednesday at 3.05 pm US time. Winds of 241km/h were recorded as it hit. If the winds had been just a few km/h stronger, it would have been a cat 5 storm.

Fort Myers, 200km south of Tampa, has a population of almost 800,000 people. Millions of Floridians are in the storm's path.

'Catastrophic impacts'

After passing over western Cuba, Ian then tracked up to the west side of Florida into the Gulf of Mexico.

Earlier on Wednesday in Tampa Bay, the water receded from the shoreline, sucked out by the hurricane. As the storm nears, all that water and more besides will surge back in potentially flooding the cities on the bay.

The US National Hurricane Centre didn't mince its words saying Ian would cause "catastrophic" storm surges, winds and flooding within the state.

• 'Nasty, nasty two days': Hurricane Ian hits Florida
• Hurricane Ian smashes into Cuba, Florida evacuating
• Hurricane Ian is headed for Florida, set to arrive within 24 hours

That sentiment was echoed by Deanne Criswell from the Federal Emergency Management Agency.

"There is going to be catastrophic impacts, and not just where we're going to see the storm make landfall, but we're also really concerned about all of the inland flooding because it's bringing with it a lot of rain and it's going to move slowly," she told CNN.

'Not going to survive'

Michael Brennan, acting deputy director of the National Oceanic and Atmospheric Administration, was even blunter as he warned of storm surges of up to 5m around the Fort Myers area.

"I'm six feet tall. That's almost three times my height," he said.

"It's not just the rise of the water from the storm surge, it's the breaking waves on top of it that are going to be driven by those 250km/h winds.

"Those waves can destroy buildings. That's not a situation you're going to survive in," said Brennan.

Ian's eye dwarfs other storms

Pictures of the eye of the storm have rattled forecasters. Usually the eye – the heart of the storm – is relatively small.

Not Ian.

Its eye is around 56km wide. Hurricane Charley, which hit Florida in 2004 and killed 10 people, had an eye of just 11km wide before it struck land.

Indeed, the extant of Hurricane Charley could fit within just the eye of Hurricane Ian.

While the eye itself is an area of relative calm, the eye wall that surrounds it is a ring of powerful thunderstorms and destructive winds. A large eye wall, as in Ian's case, means those devastating storms will take longer to pass.

Ian's eyewall underwent a "replacement" over the last 12 hours. That occurs when a larger wall of thunderstorms surrounds and replaces the earlier, smaller eye wall.

CCTV cameras in Fort Myers are mounted several metres off the ground and have been swamped with water. While in nearby Naples, a shark was seen swimming down a flooded suburban street.

Almost 700,000 people are without power.

Ian is expected to curve its way north through the state and then into the Atlantic. But it's possible it could then make landfall again around Georgia or South Carolina.

Airports in Tampa, Miami and Orlando have stopped commercial flights.

On Tuesday, Ian 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, reported AFP.

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.

- Benedict Brook, news.com.au