Photo taken on Sept. 28, 2022 shows flooded streets in Naples, Florida, the United States. (City of Naples government/Handout via Xinhua)

"The amount of water that's been rising and will likely continue to rise today, even as the storm is passing, is basically a 500-year flood event," Florida Governor Ron DeSantis said.

Ian has caused significant damage to Florida, the southeasternmost U.S. state, and a hurricane warning was issued for the entire coast of South Carolina on Thursday.

The U.S. National Hurricane Center (NHC) said in its latest advisory issued late Thursday morning that Ian is now a tropical storm with maximum sustained winds of 110 kilometers per hour, which is moving north-northeast at 15 kilometers per hour.

The storm, which made landfall in southwest Florida as Category 4 hurricane on Wednesday afternoon, is forecast to regain hurricane strength on Thursday evening and make landfall in South Carolina on Friday.

Coastal areas from Savannah River to Little River Inlet in South Carolina are under the hurricane warning.

The alert means that hurricane conditions are expected somewhere within the warning area. Preparations to protect life and property should be rushed to completion.

"Major-to-record river flooding" will continue across central Florida through next week, according to the NHC advisory.

Considerable flash, urban, and river flooding is expected across coastal portions of northeast Florida, southeastern Georgia, and eastern South Carolina through Friday.

"The amount of water that's been rising and will likely continue to rise today, even as the storm is passing, is basically a 500-year flood event," Florida Governor Ron DeSantis told reporters on Thursday morning.

DeSantis added the damage caused by the storm to Florida will likely take years to repair.

"You're looking at a storm that's changed the character of a significant part of our state," he said. "This is going to require not just emergency response now, in the days or weeks ahead. I mean, this is going to require years of effort to be able to rebuild and to come back."

The Sanibel Causeway, which connects Sanibel Island to Florida's mainland, has been damaged with portions washed away by storm surges.

One fatality related to Ian has been reported in Osceola County, Florida.

As of Thursday noon, more than 2.6 million customers are without power in Florida due to the impact of the destructive storm, according to PowerOutage.us.

U.S. President Joe Biden spoke with DeSantis on Thursday morning about the federal assistance to the state in response to Ian, according to the White House.

Biden declared a major disaster exists in Florida earlier in the day and he and DeSantis committed to continued close coordination.