Hurricane Milton marched across central Florida on Thursday after making landfall on the state's west coast hours earlier, whipping up deadly tornadoes, destroying homes and knocking out power to nearly two million people.

The storm made landfall this morning Hong Kong time as a Category 3 hurricane with maximum sustained winds of 195 kph near Siesta Key, the US National Hurricane Centre said.

By early Thursday, wind speeds reduced to a still dangerous 150 kph, dropping Milton to a Category 1 hurricane, with heavy rains and damaging storm surges.

The hurricane was located about 75 kilometres west-southwest of Cape Canaveral, home to NASA's Space Force Station.

A flash flood emergency was in effect for the Tampa Bay area including the cities of Tampa, St. Petersburg and Clearwater, the hurricane centre said, with St. Petersburg already receiving 422 millimetres of rain on Wednesday.

The eye of the storm made landfall in Siesta Key, a barrier island town of some 5,400 off Sarasota about 100 kilometres south of the Tampa Bay metropolitan area, which is home to more than three million people.

Florida Governor Ron DeSantis said he hoped Tampa Bay, once seen as the potential bull's eye, could dodge major damage and that the worst of the predicted storm surge could be avoided thanks to the landfall coming before the high tide.

Forecasters said seawater could still rise as high as four metres.

DeSantis reported Milton had also spawned at least 19 tornadoes caused damage in numerous counties, destroying around 125 homes, most of them mobile homes.

"At this point, it's too dangerous to evacuate safely, so you have to shelter in place and just hunker down," DeSantis said upon announcing the landfall.

At least two deaths were reported at a retirement community following a suspected tornado in Fort Pierce on the eastern coast of Florida, NBC News reported, citing St. Lucie County Sheriff Keith Pearson.

Pearson estimated 100 homes were destroyed in the county where some 17 tornadoes touched down, NBC said.

More than two million homes and businesses in Florida were without power, according to PowerOutage.us.

The storm was expected to cross the Florida peninsula overnight and emerge into the Atlantic, still with hurricane force, on Thursday. (Reuters)