Hurricane Ian intensified to just shy of catastrophic Category 5 strength Wednesday as its heavy winds began pummelling the US state of Florida, with forecasters warning of life-threatening storm surges after leaving millions without power in Cuba.

Mandatory evacuation orders had been issued in a dozen coastal Florida counties, with voluntary evacuation recommended in several others, according to the state's emergency officials as they girded for a potentially historic storm.

In a pre-dawn advisory the US National Hurricane Center (NHC) said "Ian has strengthened into an extremely dangerous Category 4 hurricane," warning later of "catastrophic storm surge, winds, and flooding."

At 7:00 am (1100 GMT) it said "data from a Hurricane Hunter aircraft indicate that maximum sustained winds have increased to near 155 miles (250 kilometers) per hour" -- just shy of Category 5, the strongest category on the Saffir-Simpson scale.

"This is going to be a nasty, nasty day, two days," Governor Ron DeSantis said early Wednesday as he warned residents of a "rough stretch" ahead for Florida.

"It could make landfall as a Category 5, but clearly this is a very powerful major hurricane that's going to have major impacts, both on... southwest Florida but as it continues to work through the state."

The NHC for its part said Ian was "rapidly intensifying," while conditions along the Florida coast were "rapidly deteriorating."

Tropical storm-strength winds were already battering the Florida Keys, as the storm was expected to make landfall later Wednesday near Fort Myers and Port Charlotte, along the state's west coast, before moving across central Florida and emerging in the Atlantic Ocean by late Thursday.

With up to two feet (61 centimeters) 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 16 feet (3.6 to 4.9 meters) above ground, authorities were warning of catastrophic conditions.

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

DeSantis said on Tuesday night that there had already been at least two "radar-indicated tornadoes" in the state, and warned those in areas projected to be hit hardest that their "time to evacuate is coming to an end."

Calls to heed evacuation warnings were echoed by US President Joe Biden, who earlier said Ian "could be a very severe hurricane, life-threatening and devastating in its impact."

White House Press Secretary Karine Jean-Pierre said Biden had spoken with DeSantis -- a potential 2024 election challenger -- Tuesday evening to discuss storm preparations.

Widespread blackout

Ian plunged all of Cuba into darkness on Tuesday 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 Insmet meteorological institute said.

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 access to 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.

In the western city of Pinar del Rio, AFP footage showed downed power lines, flooded streets and damaged rooftops.

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

About 40,000 people were evacuated across Pinar del Rio province, which bore the brunt of the storm, local authorities said.

Cuban residents described "destruction" and posted images on social media of flooded streets and felled trees.

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

In Consolacion del Sur, southwest of Havana, 65-year-old Caridad Fernandez said her roof was seriously damaged and water came through her front door.

"Everything we have is damaged," she said. "But we'll get through this, we'll just keep moving forwards."

'Life and death'

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

Authorities in several municipalities were distributing sandbags to help residents protect their homes from flooding.

Tampa International Airport suspended operations from Tuesday at 5 pm.

NASA, on the state's east coast, also took precautions, rolling back its massive Moon rocket into a storage hanger for protection.

Like DeSantis, FEMA administrator Deanne Criswell highlighted the danger of storm surge, saying it was the agency's "biggest concern."

"If people are told to evacuate by their local officials, please listen to them. The decision you choose to make may be the difference between life and death," she said.