There are fears "hundreds" may have died in Florida after Hurricane Ian swept through the US state on Wednesday.

The state's Governor, Ron DeSantis, said the storm was "historic" and a "once in 500 year" flood event was now taking place.

The storm, which made landfall at category 4 strength with winds gusting up to 250km/h, is now heading into the Atlantic Ocean off Florida's east coast.

If the winds had been just 3km/h stronger, it would have been a category 5 storm.

It has left a trial of destruction through central Florida from Fort Myers to Daytona Beach via Orlando.

'Fatalities are in the hundreds'

Ian has now been downgraded to a tropical storm. However it's likely to strengthen and make landfall again as soon as Friday.

On Thursday morning, US time, a senior police officer covering Fort Myers in Florida's west, where the hurricane made landfall at full force on Wednesday, warned there could be many deaths in the city.

Zuram Rodriguez surveys the damage around her mobile home in Davie, Florida., early September 28, 2022. Photo / AP

"This is a life-changing event for all of us", Lee County Sheriff Carmine Marceno told ABC television's Good Morning America.

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"I don't have confirmed numbers — I definitely know the fatalities are in the hundreds".

Marceno added "thousands" of people were waiting to be rescued in the city of 800,000 people which took an almost direct hit from Ian.

"We can't access people that's the problem."

'Devastation is an understatement'

Images from the stricken city have shown flooding and boats smashed into streets. A storm surge 3.7 metres high swept through Fort Myers, strong enough to push homes off their foundations.

"To say its devastation would be a severe understatement," Fort Myers local councillor Dan Allers told the BBC.

A McLaren P1 luxury car, which according to the @lambo9286 Instagram account only had 300 miles on the clock last week, floated out in flood waters in Florida due to Hurricane Ian. Picture: Ernie/@lambo9286

"Just watching people's belongings and homes and things float by – it was a very tough scene to witness."

At least one death has been confirmed. Police in Lake Bethel, north east of Orlando, said a 72-year-old man died after heading outside to drain his pool, Fox News reported.

"While searching for him, deputies found his (torch), then spotted the victim unresponsive in a canal behind the home," police said.

It's though the slipped on an embankment and fell into the waterway.

The United States' National Hurricane Centre has said the "danger of life-threatening storm surge" remains in place for Thursday and Friday across the south eastern states".

"Widespread, life-threatening catastrophic flooding, with major to record flooding, will continue today …. through the end of the week," it said in a statement.

Around 2.5 million Floridians were without electricity on Thursday as the storm ripped down power lines.

US President Joe Biden has declared a major disaster in Florida and has released federal funds to help on the ground.

On Thursday, Governor De Santis said the storm's impact was "historic".

"We've never seen a flood event like this. We've never seen a storm surge of this magnitude.

After damaging parts of Cuba and leaving the country dark, more destruction was expected as Hurricane Ian reached Florida's western coast. Photo / Nasa Earth Observatory

"It's a once in 500 year flood event.

"It's going to end up doing extensive damage to a lot of people's homes."

Storm expected to smash land again

The storm is expected to leave Florida on Thursday morning.

Heading over water it is likely to re-energise into hurricane strength before moving north putting it on a collision course for the Georgia and South Caroline coastlines.

It could reach those two states as early as Friday afternoon (Saturday morning, AEST). Areas that could be hit include Savannah and Charleston.