The National Hurricane Centre in Miami said Helene struck near the mouth of the Aucilla River in the Big Bend area of Florida's Gulf Coast at around 11.10pm local time.

Winds in excess of 140mph (225kph) and flash floods were reported.

Florida Governor Ron DeSantis told reporters one person had died while driving on a motorway when a sign fell on to their car.

"When Floridians wake up tomorrow morning, we're going to be waking up to a state where, very likely, there's been additional loss of life. And certainly, there's going to be loss of property," Mr DeSantis said.

"You're going to have people that are going to lose their homes because of this storm. So please keep those folks in mind, keep them in your prayers."

Two other people were reportedly killed in a possible tornado in neighbouring Georgia as the storm approached, the Associated Press reported.

A fourth person died in Charlotte, North Carolina, when a tree fell on a home.

Meanwhile, residents who decided not to evacuate in Florida's rural Taylor County were issued with morbid advice by officials.

The area's sheriff's office said in a Facebook statement: "Please write your name, birthday, and important information on your arm or leg in a PERMANENT MARKER so that you can be identified and family notified."

'Unsurvivable scenario' to play out

More than one million homes and businesses were already without power shortly after the hurricane made landfall, according to tracking website poweroutage.

States of emergency have been declared in Florida, Georgia, and the Carolinas, with hurricane and flash flood warnings in place as far away as south-central Georgia.

Officials pleaded with residents in the path of the storm to heed mandatory evacuation orders or face life-threatening conditions.

The surge caused by the hurricane - the wall of seawater pushed on land by hurricane-force winds - could rise as high as 20ft (6.1m) in some spots, as tall as a two-storey house, Michael Brennan, director of the hurricane centre, said in a video briefing.

"A really unsurvivable scenario is going to play out" in the coastal area, Mr Brennan said, with water capable of destroying buildings and carrying cars pushing inland. Millions of people are under the current flood watch.

Forecasters warned the storm surge could be particularly "catastrophic and unsurvivable" in Apalachee Bay. Officials there described the situation as a "nightmare".

'It's going to cause a lot of damage'

Residents in the city of Tallahassee told Sky's US partner NBC News that they stocked up on sandbags, food and supplies, before leaving their homes.

City mayor John Dailey urged people to take the evacuation warnings "extremely seriously", calling Helene "the biggest storm in the history of the city to hit us head-on".

Speaking to NBC News earlier this week, Mr Dailey said though they are "very prepared", he was also "very nervous, and I hope everyone is nervous".

He added: "This is a big storm. It is going to cause a lot of damage."

Jared Miller, sheriff of Wakulla County, went further - calling the storm "not a survivable event for those in coastal or low-lying areas".

The county has issued a mandatory evacuation order, but one resident, Christine Nazworth from Crawfordville, which is located about 25 miles (40km) from Apalachee Bay, said her family would be sheltering in place.

She said: "I'm prayed up. Lord have mercy on us. And everybody else that might be in its path."

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Leslie Powell, from Quincy, a city a similar distance from Tallahassee, told NBC News she was leaving her mobile home to go to a shelter with her eight-month-old baby and six-year-old daughter.

She said simply: "I'm scared. I've got a lot of trees around my home, so it's not safe for me and my kids."

Later, soon after it crossed over land, Helene weakened to a tropical storm over Georgia, with its maximum sustained winds falling to 70mph (110kph).