FLORIDA, United States (AFP) — Hurricane Ian slammed into the coast of south-west Florida as a monster Category 4 storm on Wednesday with powerful winds and torrential rains threatening “catastrophic” damage and flooding.

The National Hurricane Center (NHC) said the eye of the “extremely dangerous” hurricane made landfall just after 3:00 pm on the barrier island of Cayo Costa, west of the city of Fort Myers.

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

Fort Myers, which has a population of more than 80,000, was also experiencing severe coastal flooding with some neighbourhoods resembling lakes.

The NHC said Ian was packing maximum sustained winds of 150 miles (240 kilometres) per hour when it made landfall and forecast “catastrophic storm surge, winds and flooding in the Florida peninsula”.

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.

Ian was expected to affect several million people across Florida and in the south-eastern states of Georgia and South Carolina, and may have caused multiple casualties offshore.

The US Border Patrol said 20 migrants were missing after their boat sank. Four Cubans who survived swam to shore in the Florida Keys and three were rescued at sea by the coast guard.

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

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

Florida Governor Ron DeSantis said the state was going to experience a “nasty, nasty day, two days”.

‘Life-threatening situation’

Punta Gorda, north of Fort Myers, was being pounded by torrential rain and streets emptied as the howling winds ripped fronds off of palm trees and shook electricity poles.

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 stressed it was too late to flee and residents should hunker down and stay indoors.

With winds of 150 miles per hour as it made landfall, Ian was just seven miles per hour shy of Category 5 intensity — the strongest on the Saffir-Simpson scale.

Airports in Tampa and Orlando stopped all commercial flights and cruise ship companies delayed departures or cancelled voyages.

With up to two feet (61 centimetres) 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 (3.6 to 5.5 metres), authorities said as they warned of dire emergency conditions.

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