Hurricane Helene has weakened to a tropical storm over Georgia with maximum sustained winds of 70mph, the US National Hurricane Centre said.

Helene continues to weaken while moving further inland over Georgia.

The storm was about 40 miles east of Macon and about 100 miles south east of Atlanta, moving north at 30mph at 5am local time, the centre in Miami reported.

Surfers take advantage of heavy winds along Higgs Beach in Key West, Florida (Rob O’Neal/The Key West Citizen via AP)

It made landfall in northwestern Florida as a Category 4 storm as forecasters warned the enormous system could create a “nightmare” storm surge and bring dangerous winds and rain across much of the southeastern US.

There were at least three storm-related deaths.

The hurricane centre said Helene roared ashore at around 11.10pm local time on Thursday near the mouth of the Aucilla River in the Big Bend area of Florida’s Gulf Coast.

It had maximum sustained winds estimated at 140mph.

That location was only about 20 miles north west of where Hurricane Idalia came ashore last year at nearly the same ferocity and caused widespread damage.

Helene prompted hurricane and flash flood warnings extending far beyond the coast up into northern Georgia and western North Carolina.

More than 1.2 million homes and businesses were without power in Florida, more than 190,000 in Georgia and more than 30,000 in the Carolinas, according to the tracking site poweroutage.us.

The governors of those states and Alabama and Virginia all declared emergencies.

One person was killed in Florida when a sign fell on their car and two people were reported killed in a possible tornado in south Georgia as the storm approached.

The St Pete Pier in St Petersburg, Florida (Martha Asencio-Rhine/Tampa Bay Times via AP)

“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,” Florida governor Ron DeSantis said at a news conference on Thursday night.

Helene was moving rapidly inland after making landfall, with the centre of the storm set to race from southern to northern Georgia through early Friday morning.

The risk of tornadoes would also continue overnight and into the morning across north and central Florida, Georgia, South Carolina and southern North Carolina, forecasters said.

Later on Friday, there would be the risk of tornadoes in Virginia.

“Helene continues to produce catastrophic winds that are now pushing into southern Georgia,” the hurricane centre said in an update at 1am on Friday.

“Persons should not leave their shelters and remain in place through the passage of these life-threatening conditions.”

The hurricane’s eye passed near Valdosta, Georgia, as the storm churned rapidly north into Georgia on Thursday night.

The National Hurricane Centre issued an extreme wind warning for the area, meaning possible hurricane-force winds exceeding 115mph.