Monster Hurricane Ian brought a "500-year flood event" to Florida, devastating coastal cities, inundating homes and businesses and leaving island communities cut off, Florida Governor Ron DeSantis said Thursday.

On the morning after Ian roared ashore as one of the most powerful hurricanes to hit Florida in decades, the full extent of destruction was only beginning to emerge, but the size and ferocity of the storm stoked fears of massive devastation across swathes of the southeastern US state.

"Some of those areas -- Cape Coral, city of Fort Myers -- they got really, really inundated and really devastated by this storm," DeSantis told a press conference.

"The amount of water that's been rising, and will continue today even as the storm is passing, is basically a 500-year flood event."

Fort Myers, a city of 83,000 on Florida's southwestern coast, is surrounded by canals, inlets, and rivers. Much of the city was overwhelmed by several feet of storm surge from the Gulf of Mexico.

Nearby Pine Island and Sanibel Island, popular with vacationers, saw their causeways to the mainland badly damaged.

The two bridges from the islands to Fort Myer "are not passable" and will require structural rebuilds, DeSantis said.

"The coast guard has been performing rescue missions on the barrier islands consistently since the wee hours of the morning," he added.

The National Hurricane Center downgraded Ian, which came ashore as an extremely dangerous Category 4 hurricane at about 3:00 pm (1900 GMT) Wednesday, to a tropical storm early Thursday as it moved inland.

But the NHC also warned of severe danger from flooding and monsoon-like rains in central Florida.

DeSantis said more than two million homes and businesses were without power, especially in hard-hit Lee County where Fort Myer is located, saying it was "off the grid."

He also expressed caution over reports of several dead in Lee County, saying "we have had two unconfirmed fatalities."

Ian's wrath was having broad impacts across the state.

"I think we've never seen a flood event like this," DeSantis said. "We've never seen storm surge of this magnitude."