U.S. emergency crews tried on Thursday to reach stranded Florida residents as a resurgent Hurricane Ian veered toward the Carolinas after cutting a path of destruction across Florida, leaving behind deadly floodwaters, downed power lines and widespread damage.

Ian, one of the strongest storms ever to hit the U.S. mainland, flooded Gulf Coast communities and knocked out power to millions before plowing across the peninsula to the Atlantic Ocean, where it regained strength ahead of another anticipated landfall in South Carolina on Friday afternoon.

Florida's death toll remained uncertain amid scattered reports of casualties. At an evening news briefing, Governor Ron DeSantis acknowledged some people had perished but warned against speculating before official confirmation.

"We fully expect to have mortality from this hurricane," DeSantis said.

President Joe Biden, speaking earlier at Federal Emergency Management Agency (FEMA) headquarters in Washington, said Ian could prove to be the deadliest in state history.

"The numbers are still unclear, but we're hearing early reports of what may be substantial loss of life," Biden said.

More than 2.3 million homes and businesses in Florida remained without power, according to tracking website PowerOutage.us.

Georgia, South Carolina and North Carolina were bracing for impact as Ian still boasts of maximum sustained wind speeds of 120 kilometers per hour, according to the U.S. National Hurricane Center.

North Carolina Governor Roy Cooper urged residents to "take necessary precautions," warning of possible flooding, landslides and tornadoes.

"This storm is still dangerous," Cooper said.

Ian blasted ashore in Florida at the barrier island of Cayo Costa on Wednesday afternoon as a Category 4 hurricane with maximum sustained winds of 241 kilometers per hour.

The storm transformed Florida's southwestern shoreline, dotted with sandy beaches, coastal towns and mobile home parks, into a disaster zone as Ian swept seawater into waterfront homes.

There had been at least 700 confirmed rescues as of Thursday evening, DeSantis said. Most schools will reopen on Friday or next Monday.

Biden spoke to DeSantis on Thursday, and FEMA Director Deanne Criswell will be in Florida on Friday. Biden said that he will travel to the state when conditions allow.

Biden also approved a disaster declaration, making federal resources available to the counties impacted by the storm.

(With input from Reuters)