Tampa, Florida - At least 40 people have died after Hurricane Helene slammed into the southeastern US as a dangerous Category 4 storm, before later weakening to a tropical depression by Friday afternoon.

The deaths were reported in US media across the states of Georgia, North Carolina, South Carolina, and Florida. The governor of Georgia said at least 11 people had died in his state.

Around 4 million homes and businesses were without power across those states and beyond, with houses destroyed and entire communities flooded.

There were fears that many people could still be trapped under collapsed and damaged buildings. Many roads across the region were not passable.

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Vice President Kamala Harris said she and President Joe Biden were monitoring the situation closely.

She said some 1,500 personnel had been deployed to support impacted communities.

The National Weather Service said Helene made landfall on the west coast of Florida as a Category 4 hurricane – the second highest category – but then began losing steam.

The service had warned of "historic and catastrophic flooding," and in some places, on Friday, the water was over 16 feet high. There were also threats of mudslides due to the extreme rain.

Many victims were killed or injured by falling trees, while others died in their cars. In Florida, a woman drowned in her home, CNN reported.

In Tennessee, some 50 patients and staff were stranded on the roof of a hospital in the town of Erwin. Senator Bill Hagerty said the group was later airlifted from the roof of the Unicoi County Hospital.

In North Carolina, authorities warned that a dam could break and urged nearby residents to move to safety.

After pummelling the southeast, it began moving over the Appalachian mountains and affecting states like Tennessee and Virginia.

Helene had already caused flooding and power outages in Cuba as it barrelled toward the US.

Tropical cyclones form over warm ocean waters. Experts say global warming increases the probability of severe storms.

The hurricane season begins in the Pacific on May 15 and in the Atlantic on June 1. The season ends in both regions on November 30.