Helene hit Florida's Big Bend region as a powerful Category 4 hurricane. (AP PHOTO)

Tropical Depression Helene has brought life-threatening flooding 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 four million homes and businesses without power.

In Tennessee, fears that a dam would fail near the city of Newport prompted officials to order the evacuation of the downtown area. Another dam in North Carolina was on the brink of failure.

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 packing 225km/h winds. It left behind a chaotic landscape of overturned boats in harbours, felled trees, submerged cars and flooded streets.

As of early Friday afternoon, the storm had been downgraded to a tropical depression with maximum sustained winds of 55km/h, 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.

Elsewhere in Tennessee, Rob Mathis, the mayor of Cocke County, wrote on social media that the Walters dam “has suffered a catastrophic failure” and that the downtown area of the nearby city of Newport, with 36,000 people, would evacuate.

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.”

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

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, describing the storm surge as “unsurvivable,” as NHC Director Michael Brennan warned.

Two other people in Florida died, Governor Ron DeSantis confirmed. Georgia Governor Brian Kemp cited 11 storm-related fatalities in his state so far, while North Carolina Governor Roy Cooper said there had been two deaths there.

At least 13 people had 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 500km, according to the National Hurricane Center. By comparison, Idalia, another major hurricane that struck Florida’s Big Bend region in 2023, had tropical-storm winds extending 260km about eight hours before it made landfall.

More than 4.6 million homes and businesses were without power on Friday in Florida, Georgia, the Carolinas and other states.