Hurricane Helene has weakened to a Category 1 hurricane with maximum sustained winds of 120 kph (75 mph), according to the National Hurricane Centre, but rescue teams and emergency response units are still hard at work as the Florida and Georgia residents deal with the damage.

An airboat was being used to help people on flooded highways in the Floridian city of Crystal River after Helene roared through the state.

Citrus County Sheriff’s Office urged people to stay off the roads, saying in a post that first responders were out in boats rescuing people trapped in floodwaters.

The storm traveled north from Florida to Georgia and was about 160km (100 miles) from Augusta and 65km (40 miles) from Macon moving at about 48 kph (30 mph), the hurricane center in Miami said in an update.

Airboats are being used for rescues on highways flooded in Florida. - Citrus County Sheriff's Office via Reuters

Hurricane Helene made landfall in northwestern Florida as a Category 4 storm as forecasters warned that 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.

Helene hit Florida packing sustained winds of around 130mph (209km/h), the National Hurricane Center said, making it a powerful Category 4 storm. Even before it made landfall, the storm had flooded the Gulf Coast and knocked out power for at least 1 million customers in the state.

The National Hurricane Center in Miami said Helene roared ashore around 11:10pm 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 225 kph (140 mph). That location was only about 32km  (20 miles) northwest of where Hurricane Idalia came ashore last year at nearly the same ferocity and caused widespread damage.

Storms cause by Hurricane Helene have triggered flooding in Naples, Florida. - Greg Dyer via Reuters

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.

“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.

Hurricane Helene. – AP

Helene's surge – the wall of seawater pushed on land by hurricane-force winds – could rise to as much as 20 feet (6.1m) in some spots, as tall as a two-storey house, the centre's director Michael Brennan said in a video briefing.

"A really unsurvivable scenario is going to play out (in the coastal area)," Brennan said.

He said water capable of destroying buildings and carrying cars would push inland.

Strong rain bands were whipping parts of coastal Florida, and rainfall had already lashed Georgia, South Carolina, central and western North Carolina and portions of Tennessee. Atlanta, hundreds of miles north of Florida's Big Bend, was under a tropical storm warning.

Florida Governor Ron DeSantis told reporters the hurricane had already caused one fatality. He gave no details.

Strong winds already cut power to more than 600,000 homes and businesses in Florida, according to the tracking site poweroutage.us. The governors of Florida, Georgia, Alabama, the Carolinas and Virginia all declared emergencies in their states.

The National Weather Service in Tallahassee issued an “extreme wind warning” for the Big Bend as the eyewall approached.

“Treat this warning like a tornado warning,” it said. “Take shelter in the most interior room and hunker down.”

Helene arrives barely a year since Hurricane Idalia slammed into Florida’s Big Bend and caused widespread damage.

Storm surge from Hurricane Helene could be devastating. – AP

The storm’s wrath is being 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. Lumber and other debris from a fire in Cedar Key a week ago crashed ashore in the rising water.

The Category 4 storm is also expected to produce “catastrophic winds” in the area over the next few hours, the centre said.

“This is an extremely dangerous and life-threatening situation,” the NHC said.

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

The centre warned when the storm’s eye comes ashore, conditions will temporarily calm, but the eye’s other side will quickly arrive and produce hazardous winds again.

Watch Hurricane Helene as seen from the International Space Station. – AP

A tornado watch was issued late Thursday for much of Florida and southeastern Georgia, the National Weather Service in Tallahassee said.

Florida’s Gulf Coast, including Tampa and the Big Bend area,  are under the watch, as was the state’s Atlantic coast, including Orlando. The tornado watch will be in effect through early Friday morning.

In Pinellas County, which sits on a peninsula surrounded by Tampa Bay and the Gulf of Mexico, roads were already filling with water before noon. Officials warned the storm's impact could be as severe as last year's Hurricane Idalia, which flooded 1500 homes in the low-lying coastal county.

Videos posted on the county's social media site showed some swamped beachside roads and water rising over boat docks.

Airports in Tampa, Tallahassee and St Petersburg all suspended operations on Thursday.

Florida braces for Hurricane Helene. – Reuters

DeSantis earlier warned North Florida residents to flee before time ran out.

"You have time to get to a shelter, but you've got to do it now," he said at a morning news briefing.

"Every minute that goes by brings us closer to having conditions that are going to be simply too dangerous to navigate."

Helene is expected to remain a full-fledged hurricane as it rolls through the Macon, Georgia, area on Friday, forecasters said. It could bring 12 inches (30.5 cm) of rain or more, potentially devastating the state's cotton and pecan crops, which are in the middle of harvesting season.

"The current forecast for Hurricane Helene suggests this storm will impact every part of our state," Georgia Governor Brian Kemp said.

After making landfall across the Florida coast, Helene is expected move more slowly over the Tennessee Valley on Friday and Saturday, the NHC said.

The National Weather Service said more than a dozen Georgia counties could see winds exceeding 110mph (177km/h), including Lowndes County where Valdosta is located along Interstate 75.

Other inland cities in rural southern Georgia where the hurricane threat was highest included Thomasville, Moultrie and Tifton.

President Joe Biden declared a state of emergency in Alabama.

Wall of water 

Storm surge was forecast to reach 15 to 20 feet (4.6m to 6.1m) in the Big Bend area of Florida's Panhandle region where the storm is expected to come ashore.

Numerous evacuations were ordered along Florida's Gulf Coast, including Sarasota and Charlotte counties.

Georgia governor warns residents of Hurricane Helene's impact. – Reuters

Pinellas County officials ordered evacuations of long-term healthcare facilities near the coast, including nursing homes, assisted living centres and hospitals.

Not everyone heeded the evacuation orders. In coastal Dunedin, Florida, about 25 miles west of Tampa, state ferry boat operator Ken Wood, 58, planned to ride out the storm with his 16-year-old cat, Andy.

"We're under orders, but I'm going to stay right here at the house," Wood told Reuters by telephone.

"The storm looks like it'll be a bit west of us, but who knows? I'm sure it'll be interesting, to say the least."

Tallahassee officials expressed concern that the storm could cause unprecedented damage.

Reinsurance broker Gallagher Re said preliminary private insurance losses could reach $3 billion to $6 billion, with additional losses to federal insurance programs approaching a potential $1 billion.

Energy facilities along the US Gulf Coast scaled back operations and evacuated some production sites.

The director of the Federal Emergency Management Agency, Deanne Criswell, said at a White House briefing that she would travel to Florida on Friday to assess the damage.

Rain from Hurricane Helene floods parts of Cuba as it moves toward Florida. – AP

Helene was expected to dump up to 15 inches (381mm) of rain in some isolated spots after making landfall in Florida, causing considerable flash and urban flooding, the hurricane centre said.

National Hurricane Center deputy director Jamie Rhome said about half of lives lost in hurricanes typically came from flash flooding caused by torrential rain, often among people who drive into flooded roads and are swept away.

Rhome added that the expected hurricane-force wind impact area stretched around 180 miles (290km) north from the Florida panhandle to southern Georgia.

"You need to prepare for prolonged (energy) outages. Those trees are going to come down in strong winds, block roads," Rhome said.