Hurricane Ian strengthened to a Category 4 storm as it headed towards the US state of Florida on Wednesday, with forecasters warning of life-threatening storm surges and "devastating" winds after it reportedly killed two and left millions without power in Cuba.

As of 5 am (0900 GMT), 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.

In an advisory issued around the same time, the US National Hurricane Center (NHC) said "Ian has strengthened into an extremely dangerous Category 4 hurricane."

"Very recent data from an Air Force Reserve Hurricane Hunter aircraft indicate that the maximum sustained winds have increased to near 140 mph (220 km/h) with higher gusts," the NHC said.

The storm was expected to make landfall later on Wednesday before moving across central Florida and emerging in the western Atlantic by late Thursday.

The NHC said earlier that a "life-threatening storm surge is expected along the Florida west coast and the Lower Florida Keys," with "devastating wind damage" expected near Ian's core.

"Catastrophic flooding is expected across portions of central Florida with considerable flooding in southern Florida, northern Florida, southeastern Georgia and coastal South Carolina," it said.

Florida Governor Ron 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."

"You need to evacuate now. You're going to start feeling major impacts of this storm relatively soon," he said.

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 -- on Tuesday evening to discuss preparations for the storm.

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 a scattering of 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.

Two dead

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

At the time of impact, the NHC reported Ian's maximum wind speeds at 125 miles (205 kilometers) per hour.

At least two people have been reported dead in Pinar del Rio province, according to Cuban state media.

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."

In San Juan y Martinez, a hub for Cuba's vital cigar industry, "it was apocalyptic, a real disaster," said Hirochi Robaina from the Robaina tobacco plantation.

'Life and death'

In Florida, 30-year-old Chelsea Thompson was helping her parents board up their home on Tuesday in a mandatory evacuation zone southwest of Tampa, saying that "the closer it gets, obviously with the unknown, your anxiety gets a little higher."

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 free sandbags to help residents protect their homes from flooding.

Tampa International Airport suspended operations from Tuesday at 5 pm.

Biden has preemptively approved emergency aid in Florida through the Federal Emergency Management Agency (FEMA).

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.

© Agence France-Presse