Hurricane Helene makes landfall in north-western Florida as ...

Hurricane Helene has made landfall in north-western Florida as a category four storm as forecasters warned that the enormous system could create a "nightmare" storm surge and bring dangerous winds and rain across much of the south-eastern US.

The National Hurricane Center in Miami said the hurricane roared ashore about 11:10pm, local time, near the mouth of the Aucilla River in the Big Bend area of Florida's Gulf Coast.

It had maximum sustained winds estimated at 225 kilometres per hour.

At least one fatality in Florida was blamed on the hurricane so far, authorities said.

Florida Governor Ron DeSantis said the fatality was a driver whose car was struck by debris.

"When we wake up tomorrow morning, the chances are there will likely have been more fatalities," the governor added.

Authorities added it would likely be several hours before any rescue personnel could head out to help those in need.

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

Two Tampa residents hold hands as they cross a flooded street together.  (AP: Jefferee Woo/Tampa Bay Times )

Before it made landfall, strong winds had already cut power to nearly 900,000 homes and businesses in Florida, according to the tracking site poweroutage.us.

The governors of Florida, Georgia, Alabama, the Carolinas and Virginia have all declared emergencies in their states.

Helene is tied as the 14th most powerful hurricane to hit anywhere in the United States since records began, and is the seventh most powerful to slam into Florida, according to data from the National Hurricane Center.

Two people were reported killed in a possible tornado in south Georgia as the storm approached.

The National Weather Service in Tallahassee had issued an "extreme wind warning" for the Big Bend as the centre of the system approached: "Treat this warning like a tornado warning," it said in a post on X.

"Take shelter in the most interior room and hunker down!"

Helene arrived barely a year after Hurricane Idalia slammed into Florida's Big Bend and caused widespread damage. Idalia became a category four in the Gulf of Mexico but made landfall as a category three near Keaton Beach, with maximum sustained winds near 205kph.

A moving satellite image of a hurricane moving over the coast of Florida. (NOAA)

Helene's wrath was felt widely, with sustained tropical-storm-force winds and hurricane-force gusts along Florida's west coast. Water lapped over a road in Siesta Key near Sarasota and covered some intersections in St. Pete Beach.

Debris from a fire in Cedar Key a week ago crashed ashore in the rising water.

Beyond Florida, up to 25 centimetres of rain had fallen in the North Carolina mountains, with up to 36cm more possible before the deluge ends, setting the stage for flooding that forecasters warned could be worse than anything seen in the past century.

A satellite image shows Hurricane Helene churning through the Gulf of Florida.  (Reuters: NASA/Handout )

Heavy rains began falling and winds were picking up earlier on Thursday in Valdosta, Georgia, near the Florida state line. The weather service said more than a dozen Georgia counties could see hurricane-force winds exceeding 17kph.

In south Georgia, two people were killed when a possible tornado struck a mobile home on Thursday night, Wheeler County Sheriff Randy Rigdon told WMAZ-TV. 

The damage was reported as heavy thunderstorms raked much of the state. Wheeler County is about 113km south-east of Macon.

AP