Hurricane Ian unleashed "historic" devastation in Florida, flooding homes, washing out roads and bridges and leaving millions without power, officials said on Thursday, as President Joe Biden warned of a potentially "substantial" death toll.
The storm, one of the most powerful ever to hit the United States, churned towards South Carolina as a Category 1 hurricane after hammering Florida, where at least eight people died and many awaited rescue in flooded homes.
"This could be the deadliest hurricane in Florida history," Biden said after a briefing at FEMA emergency management headquarters in Washington.
He said the numbers "are still unclear, but we're hearing reports of what may be substantial loss of life."
Florida Governor Ron DeSantis described the destruction in the southwest part of the southern coastal state as a "500-year flood event."
"The damage that was done has been historic and this is just off initial assessments," DeSantis said. "We've never seen storm surge of this magnitude."
Fort Myers, where Ian made landfall as a powerful Category 4 hurricane on Wednesday, took much of the brunt of the storm as streets became rivers and sea water poured into houses.
Dozens of boats moored in the marina were sunk while others were tossed on to downtown streets.
Trees were toppled by the howling winds of up to 240 kilometers per hour.
Tom Johnson, 54, of Fort Myers had a front row seat to the destruction from his apartment on the second floor of a two-storey harbourside building.
"I was scared because I've never been through that," Johnson said. "It was just the most horrifying sounds with debris flying everywhere, doors flying off."
His home was undamaged but one of his neighbours, Janelle Thil, 42, was not so lucky and had to ask neighbours for help after her ground floor apartment began to flood.
"They got my dogs and then I jumped out of the window and swam," Thil said.
When Thil returned to her apartment after the storm passed, she said she opened the door and "had to wait about five minutes for all the floodwaters to come out."
"I loved my home," she said. "But I'm alive and that's what matters."
DeSantis, the governor, declined to give casualty estimates but county officials confirmed at least eight deaths with the number expected to rise.
The US Coast Guard said nine migrants have been rescued from a boat that sank at sea during the hurricane on Wednesday, leaving 18 missing. Among them were four Cubans who swam to shore in the Florida Keys.
DeSantis said rescue efforts for survivors trapped in flooded homes were going to be a "24/7 operation."
A Coast Guard official said helicopter crews were plucking people from the rooftops of homes inundated by floodwaters.
Ian was downgraded to a tropical storm overnight but the National Hurricane Centre (NHC) said it regained Category 1 hurricane strength on Thursday afternoon and issued a hurricane warning for the entire coast of South Carolina.
Biden declared a "major disaster" in Florida, a move that frees up federal funding for storm relief.
"We're continuing to take swift action to help the families of Florida," he tweeted. "I want the people of Florida to know that we will be here at every step of the way."
Much of Florida's southwest coast was plunged into darkness after the storm wiped out power.
Tracking website poweroutage.us said 2.6 million homes and businesses remained without electricity in the so-called Sunshine State on Thursday.
Two barrier islands near Fort Myers, Pine Island and Sanibel Island, popular with vacationers, were essentially cut off when the storm damaged causeways to the mainland.
Sanibel Island got "hit with really biblical storm surge," DeSantis said, and rescuers were using boats and helicopters to evacuate residents who rode out the storm.
Mandatory evacuation orders had been issued in many areas of Florida ahead of Ian, with several dozen shelters set up.
Airports stopped all commercial flights, and cruise ship companies delayed or cancelled voyages.
Before pummelling Florida, Ian plunged all of Cuba into darkness on Tuesday after downing the island's power network.
At least two people died in Pinar del Rio province, state media in the country of more than 11 million reported.
Human activity has caused life-threatening climate change resulting in more severe weather events across the globe. (AFP)