Hurricane Michael has slammed into the Florida Panhandle with terrifying winds of 250km/h, splintering homes and submerging neighbourhoods.

The most powerful hurricane to hit the continental US in nearly 50 years, it has continued its destructive charge inland across the Southeast, killing at least six.

Falling trees have killed a man in Gadsden County, Florida and a man in Iredell County, North Carolina.

Hurricane Michael has slammed into the Florida Panhandle with terrifying winds of 250km/h, splintering homes and submerging neighbourhoods. (AP)

A 11-year-old girl in Seminole County, Georgia, was also killed when a carport blew through the roof of her home.

Supercharged by abnormally warm waters of the Gulf of Mexico, the Category 4 storm crashed ashore early yesterday afternoon near Mexico Beach, a tourist town about midway along the Panhandle, a 320km stretch of white-sand beach resorts, fishing towns and military bases.

After it ravaged the Panhandle, Michael barrelled into south Georgia as a Category 3 hurricane – the most powerful ever recorded for that part of the neighbouring state.

Destruction left by Hurricane Michael View Gallery

Michael Williams, 70, waves to passing motorists while looking for food and water as downed trees prevent him from driving out of his damaged home with his family in the aftermath of Hurricane Michael in Springfield, Florida. (AP)

It later weakened to a Category 1 hurricane, and there were reports it spawned possible tornadoes in central Georgia.

In north Florida, Michael battered the shoreline with sideways rain, powerful gusts and crashing waves, swamping streets and docks, flattening trees, shredding awnings and peeling away shingles.

It set off transformer explosions and knocked out power to more than 388,000 homes and businesses.

A Panhandle man was killed by a tree that toppled on a home, Gadsden County Sheriff's Office spokeswoman Anglie Hightower said.

The most powerful hurricane to hit the continental US in nearly 50 years, it has continued its destructive charge inland across the Southeast, killing at least three. (AP)

Falling trees have killed a man in Gadsden County, Florida and a man in Iredell County, North Carolina. (AP)

But she added emergency crews trying to reach the home were hampered by downed trees and debris blocking roadways. The man wasn't immediately identified.

Damage in Panama City was extensive, with broken and uprooted trees and power lines down nearly everywhere.

Roofs were peeled off and homes split open by fallen trees. Twisted street signs lay on the ground.

Residents emerged in the early evening to assess damage when rains stopped, though skies were still overcast and windy.

Vance Beu, 29, was staying with his mother at her apartment, Spring Gate Apartments, a small complex of single-story wood frame apartment buildings.

A 11-year-old girl in Seminole County, Georgia, was also killed when a carport blew through the roof of her home. (AP)

Supercharged by abnormally warm waters of the Gulf of Mexico, the Category 4 storm crashed ashore early yesterday afternoon near Mexico Beach. (AP)

A pine tree punched a hole in their roof and he said the roar of the storm sounded like a jet engine as the winds accelerated. Their ears even popped as the barometric pressure dropped.

"It was terrifying, honestly. There was a lot of noise," Mr Beu said.

"We thought the windows were going to break at any time.

"We had the inside windows kind of barricaded in with mattresses."

Gov. Rick Scott announced soon after the powerful eye had swept inland that "aggressive" search and rescue efforts would get underway as conditions allow.

He urged people to stay off debris-littered roads.

A Panhandle man was killed by a tree that toppled on a home, Gadsden County Sheriff's Office spokeswoman Anglie Hightower said. (EPA)

"If you and your family made it through the storm safely, the worst thing you could do now is act foolishly," he said.

Michael was a meteorological brute that sprang quickly from a weekend tropical depression, going from a Category 2 on Tuesday to a Category 4 by the time it came ashore. It was the most powerful hurricane on record to hit the Panhandle.

More than 375,000 people up and down the Gulf Coast were urged to evacuate as Michael closed in.

But the fast-moving, fast-strengthening storm didn't give people much time to prepare, and emergency authorities lamented that many ignored the warnings and seemed to think they could ride it out.

BY THE NUMBERS

— Hurricane history: first Category 4 hurricane to make landfall in Florida's Panhandle since record-keeping began in 1851.

— Top winds: 250km/h, strong enough to completely destroy homes and cause weeks long power outages.

— Powerful pressure: 919 millibars minimum pressure in the eye, the third most intense hurricane landfall in the US in recorded history.

— High water: estimated peak storm surge of 2.75 metres and 4.25 metres from Mexico Beach east through Apalachee Bay, according to the National Hurricane Center.

— Storm riders: Roughly 375,000 people in Florida warned to evacuate. Many refused, including 285 people in Mexico Beach where Michael made landfall.

— Rescued: 47 helped out of hard-hit areas along Florida's coastline, and 20 people in flooded neighbourhoods in North Carolina.

— Staying safe: nearly 6700 people took refuge in 54 shelters in Florida.

— Power outages: Roughly a million customers in Georgia, Florida, Alabama and South Carolina without power.

— Food and water: two million ready-to-eat meals, 3.75 million litres of water and 40,000 4.5kg bags of ice ready for distribution in Florida.