Hurricane Helene killed at least 33 people and caused massive flooding across the southeast United States on Friday, knocking out power for millions of customers.

Roads, homes and businesses were inundated after Helene made landfall near the Florida state capital Tallahassee overnight and surged north, though it weakened to a tropical storm.

The National Hurricane Centre reported "historic and catastrophic flooding" and warned of flash floods in Georgia's largest city Atlanta, as well as in South Carolina and North Carolina.

Up to 12 inches (30 centimetres) of rain was forecast in the Appalachian mountains, with isolated spots even receiving 20 inches.

In South Carolina at least 14 people have died, including two firefighters, officials said. Four of the fatalities were related to "trees falling through the roof of the homes," said Darryl Ables, the coroner in Aiken County.

In Perry, a town near where Helene slammed into the coast as a powerful Category 4 hurricane, houses had lost power and the petrol station was wrecked.

"Once the eye got to us, that's when everything started to intensify," said Larry Bailey, 32, who sheltered in his small wooden home all night with his two nephews and sister.

"I am Floridian, so I'm kind of used to it, but it was real scary at one point. It's like, was my house gonna get blown away or not?"

Georgia Governor Brian Kemp reported 11 fatalities in his state, including an emergency responder, and he warned that the city of Valdosta had identified 115 heavily damaged structures with multiple people trapped inside.

Authorities in Pinellas County in Florida confirmed seven storm-related deaths, and one person also died in Charlotte, North Carolina when a tree fell on a home, the fire department said.

With typhoon Yagi battering Asia, storm Boris drenching Europe, extreme flooding in the Sahel, September so far has been a wet month globally.

Scientists link some extreme weather events directly to human-caused global warming, but it remains too early to draw clear conclusions about the current month.

"We have got to start wondering: is this the new normal? Is it going to happen every year?" said Curtis Drafton, a search and rescue volunteer, 48, in Steinhatchee, Florida.

"We have a lot of talk about once-in-a-lifetime storm, but we had one similar last year," he said.

"We had a 9-foot storm surge, two feet over my head plus a little bit more. This dock here got shredded."

Some residents in Atlanta used buckets to empty water out of their ground-floor windows.

More than 4.3 million homes and businesses were without power across Florida, Georgia and the Carolinas, according to tracking site PowerOutage.us.

In the impact zone, residents had been warned of "unsurvivable" storm surge.

President Joe Biden and state authorities had urged people to heed official evacuation warnings before Helene hit, though some chose stay in their homes to wait out the storm.

Florida Governor Ron DeSantis had mobilised the National Guard and ordered thousands of personnel to ready for search and rescue operations, urging residents to take precautions. (AFP)

Last updated: 2024-09-28 HKT 07:14