The remains of damaged homes are seen in Horseshoe Beach, Florida, on Wednesday. AP

Florida began assessing the damage on Wednesday from Idalia's flooding after the powerful storm inundated coastal communities and knocked out power to thousands, as the system advanced up the southeastern US coast bringing additional rain deluges and dangerous storm surge.

After roaring across the Sunshine State, Idalia barreled into neighboring Georgia and weakened to a tropical storm that nevertheless was drenching the region with up to 10 inches (25 centimeters) of rain and bringing life-threatening storm surges to coastal communities, officials said.

They described Idalia and its record-high surging waters as a once-in-a-lifetime event for the area of northwest Florida most affected.

While there were no immediately confirmed deaths, Florida Governor Ron DeSantis stressed "that very well may change," given the storm's magnitude.

State officials said first responders including search and rescue teams were operational, but warned it could take time to reach more remote areas blocked by fallen trees or high water.

A view shows a flooded road on 41st Avenue at Waterway Blvd, Isle of Palms, South Carolina, US, on Wednesday. Reuters

Idalia struck as an "extremely dangerous" Category 3 hurricane in Florida's marshy, sparsely populated Big Bend area around 7:45am (1145 GMT) on Wednesday, the US National Hurricane Center (NHC) reported.

The storm crashed ashore packing maximum sustained winds of approximately 125 miles (215 kilometers) per hour near the community of Keaton Beach, with a possible storm surge of up to 16 feet (about five meters) in some coastal areas, the NHC said.

Though Idalia weakened to a Category 1 hurricane and eventually a tropical storm with winds of 60 miles per hour, it remained dangerous as it raked over Georgia and into South Carolina.

Ben Almquist, emergency management director for Charleston, South Carolina, told CNN late Wednesday: "We still have plenty of floodwaters throughout the city right now," and some rescue operations were underway.

But Thursday morning was shaping up to be Idalia's last gasp.

Jewell Baggett walks amidst debris strewn across the yard where her mother’s home had stood. AP

"Everything should be improving once we pass the (Wednesday) midnight hour for just about everybody," Ron Morales, a meteorologist with the National Weather Service based in Charleston, told the city's The Post and Courier newspaper.

Idalia is forecast to blow out over the Atlantic later Thursday.

Agence France-Presse