ST PETERSBURG, Florida (AP) — Hurricane Ian made landfall Wednesday in southwest Florida as one of the most powerful storms ever recorded in the US, swamping city streets with water and smashing trees along the coast.

The massive storm was expected to trigger flooding across a wide area of Florida as it crawls northeastward, and at least one sheriff's office along the coast reported that it was already getting "a significant number of calls" from people trapped in their homes. The hurricane's center struck near Cayo Costa, a protected barrier island just west of heavily populated Fort Myers.

Mark Pritchett stepped outside his home in Venice around the time the hurricane came ashore about 35 miles (56 kilometres) to the south. He called it "terrifying."

"I literally couldn't stand against the wind," Pritchett wrote in a text message. "Rain shooting like needles. My street is a river. Limbs and trees down. And the worst is yet to come."

The Category four storm slammed the coast with 150 mph (241 kph) winds and pushed a wall of storm surge accumulated during its slow march over the Gulf of Mexico. The storm previously tore into Cuba, killing two people and bringing down the country's electrical grid.

About 2.5 million people were ordered to evacuate southwest Florida before the storm hit, but but by law no one could be forced to flee. Though expected to weaken as it marched inland at about 9 mph (14 kph), Ian's hurricane force winds were likely to be felt well into central Florida.

"This is going to be a nasty nasty day, two days," Florida Governor Ron DeSantis said, stressing that people in Ian's path along the coast should rush to the safest possible shelter and stay there.

Jackson Boone left his home near the coast and hunkered down at his law office in Venice with employees and their pets. Boone at one point opened a door to howling wind and rain flying sideways.

"We're seeing tree damage, horizontal rain, very high wind — I think it's a 100-plus sustained outside right now," Boone said by telephone. "We have a 50-plus-year-old oak tree that has toppled over."

In Naples, the first floor of a fire station was inundated with about 3 feet of water and firefighters worked to salvage gear from a firetruck stuck outside the garage in even deeper water, a video posted by the Naples Fire Department showed. Naples is in Collier County, where the sheriff's department reported on Facebook that it was getting "a significant number of calls of people trapped by water in their homes" and that it would prioritise reaching people "reporting life threatening medical emergencies in deep water."

Ian's windspeed at landfall tied it for the fifth-strongest hurricane to strike the US, along with several other storms. Among them was Hurricane Charley, which hit almost the same spot on Florida's coast in August 2004, killing 10 people and inflicting $14 billion in damage.