Tropical Depression Helene brought life-threatening flooding on Friday to wide sections of the US Southeast, where at least 43 people have been killed by a storm that swamped neighbourhoods, triggered mudslides, threatened dams and left more than 3.5 million homes and businesses without power.

Before moving north through Georgia and into Tennessee and the Carolinas, Helene hit Florida's Big Bend region as a powerful Category 4 hurricane on Thursday night, packing 140mph (225 kph) winds. It left behind a chaotic landscape of overturned boats in harbours, felled trees, submerged cars and flooded streets.

By early Friday afternoon, the storm had been downgraded to a tropical depression with maximum sustained winds of 35mph (55 kph), the National Hurricane Center said.

But Helene's heavy rains were still producing catastrophic flooding in many areas, with police and firefighters carrying out thousands of water rescues throughout the affected states.

More than 50 people were rescued from the roof of a hospital in Unicoi County, Tennessee, about 120 miles (200km) northeast of Knoxville, state officials said, after floodwaters swamped the rural community.

Rising waters from the Nolichucky River were preventing ambulances and emergency vehicles from evacuating patients and others there, the Unicoi County Emergency Management Agency said on social media. Emergency crews in boats and helicopters were conducting rescues.

Elsewhere in Tennessee, Rob Mathis, the mayor of Cocke County, ordered the evacuation of downtown Newport because of a potential dam failure nearby. Mathis initially said the Walters dam had suffered a "catastrophic failure" but later said the dam "was breached" but there had been no major failure as yet.

The Tennessee Emergency Management Agency wrote on social media that the Walters dam, which is located just across state lines in North Carolina, had not failed. The agency said that information came from Duke Energy, which operates the dam.

Madison McDonald, a Duke Energy spokesperson, said, "we are aware of the situation and we're sorting out the facts."

In western North Carolina, Rutherford County emergency officials warned residents near the Lake Lure Dam to immediately evacuate to higher ground, saying "Dam failure imminent."

But by 8 p.m. (0000 GMT), the county emergency officials said that engineers had evaluated the dam "and determined it is no longer at imminent risk of failure."

In nearby Buncombe County, landslides forced interstates 40 and 26 to close, the county said on X.

The extent of the damage in Florida began emerging after daybreak on Friday.

In coastal Steinhatchee, Florida, a storm surge - the wall of seawater pushed ashore by winds - of eight to 10 feet (2.4-3 meters) moved mobile homes, the National Weather Service said on X. In Treasure Island, a barrier island community in Pinellas County, boats were grounded in front yards.

The city of Tampa posted on X that emergency personnel had completed 78 water rescues of residents and that many roads were impassable because of flooding. The Pasco County sheriff's office rescued more than 65 people overnight.

Officials had pleaded with residents in Helene's path to heed evacuation orders, with National Hurricane Center Director Michael Brennan describing the storm surge as "unsurvivable."

Some residents stayed put.

Ken Wood, 58, a state ferry boat operator in Pinellas County, said he should have heeded evacuation orders rather than riding out the storm at home with his 16-year-old cat, Andy.

"I'll never do that again, I swear," Wood said. "It was a harrowing experience. It roared all night like a train. It was unnerving. The house shook."

Down the hill from his house, the storm flooded some homes with chest-deep salt water. One house caught fire and burned down, shooting 30-foot flames in the stormy sky, he said.

"Old Andy seemed like he didn't care," Wood said. "He did fine. But next time we leave."

Pinellas County Sheriff Bob Gualtieri said first responders were unable to answer several emergency calls from residents overnight due to the conditions. On Friday, county authorities found at least five people dead.

Two others in Florida died, said Governor Ron DeSantis. Georgia Governor Brian Kemp's office reported 15 storm-related fatalities in that state, while North Carolina Governor Roy Cooper said there had been two deaths there.

At least 19 people died during the storm across South Carolina, the Charleston-based Post and Courier newspaper reported, citing local officials.

Helene was unusually large for a Gulf hurricane, forecasters said, though a storm's size is not the same as its strength, which is based on maximum sustained wind speeds.

A few hours before landfall, Helene's tropical-storm winds extended outward 310 miles (500 km), according to the National Hurricane Center. By comparison, Idalia, a major hurricane that struck Florida's Big Bend region last year, had tropical-storm winds extending 160 miles (260 km) about eight hours before it made landfall.