Michael, whose rapid intensification as it churned north over the Gulf of Mexico caught many by surprise, made landfall yesterday afternoon near Mexico Beach, about 32km southeast of Panama City in Florida's Panhandle region, with top sustained winds reaching 249km/h.

The fiercest storm to hit Florida in 80 years came ashore as a Category 4 hurricane on the five-step Saffir-Simpson windscale, the biggest storm on record to strike the Florida Panhandle.

Its sustained winds were just 3.2km/h shy of an extremely rare Category 5.

The storm's intensity waned steadily as it pushed inland and curled northeasterly into Georgia.

It was downgraded to a tropical storm, with top sustained winds diminishing to 95km/h early today.

The governors of North and South Carolina urged residents to brace for more heavy rain and storm-force winds as Michael ploughs northward up the Atlantic seaboard.

The Carolinas are still reeling from severe flooding in the aftermath of Hurricane Florence less than a month ago.

The National Hurricane Center said Michael would pass through the Carolinas, dumping as much as 20cm of rain in some areas. Up to 30cm of rain was forecast in Florida.

Gadsden County sheriff's spokeswoman Anglie Hightower said a "male subject" was killed by a tree toppling onto his house in Greensboro, Florida, near the state capital, Tallahassee, in the first report of a fatality from the hurricane.

Severe flooding, heavily damaged buildings, uprooted trees and downed power lines appeared widespread in coastal areas near the storm's landfall.

Television news footage during the day showed many homes submerged in floodwaters up to their roofs in Mexico Beach, where the fate of about 280 residents who authorities said defied evacuation orders was unknown.

Numerous buildings in Panama City were demolished, partially collapsed or without roofs amid deserted streets littered with debris, twisted, fallen tree trunks and dangling wires.

Bill Manning, a 63-year-old supermarket employee, fled his camper van in Panama City for safer quarters in a hotel only to see the electricity there go out.

"My God, it’s scary. I didn’t expect all this," he said.

Without power, the city was plunged into darkness at nightfall and its flooded streets were mostly silent and devoid of people or traffic.

More than 403,000 homes and businesses were without electricity in Florida, Georgia and Alabama, utility companies said.