Hurricane Michael has been reduced to a category one hurricane, hours after it hit the Gulf Coast of Florida, leaving two people dead.
The third-most powerful storm ever to strike the US mainland has headed northeast to soak Georgia and the Carolinas, leaving the Florida Panhandle to assess the devastation left by Hurricane Michael.
A man was killed when a tree toppled onto his house in Florida and a girl died when debris fell into a home in Georgia, local media reported and officials said.
The Category 4 storm was the fiercest hurricane to hit Florida in 80 years when it came ashore as a on Wednesday, but its strength waned as it pushed into Georgia. Early on Thursday, it was downgraded to a tropical storm, with top sustained winds diminishing to 91 km/h.
More than 700,000 homes and businesses were without power in Florida, Alabama and Georgia early on Thursday. Thousands hunkered down in shelters overnight after fleeing their homes to escape the fast-approaching storm.
Michael, whose rapid intensification as it churned north over the Gulf of Mexico caught many by surprise, made landfall on Wednesday afternoon near Mexico Beach, about 32km southeast of Panama City in Florida's panhandle. Top sustained winds reached 249km/h.
The governors of North and South Carolina urged residents to brace for heavy rain and storm-force winds as Michael ploughed northward up the Atlantic seaboard. The Carolinas are still recovering from the flooding that followed Hurricane Florence less than a month ago.
The National Hurricane Centre said Michael would pass through the Carolinas on Thursday, dumping as much as 8 inches of rain in some areas. Up to 30cm of rain was forecast in Florida.
Television news footage during the day showed homes submerged in floodwaters up to their roofs in Mexico Beach. The fate of about 280 residents who authorities said ignored evacuation orders was unknown.
Numerous buildings in Panama City were demolished or left without roofs amid deserted streets littered with debris, twisted, fallen tree trunks and dangling wires.
Some 500,000 Florida residents were ordered or urged to seek higher ground before the storm in 20 counties spanning a 322km stretch of shoreline, the Federal Emergency Management Agency (FEMA) said.
An estimated 6,000 evacuees took cover in emergency shelters, most of them in Florida, and that number was expected to swell to 20,000 across five states by week's end.
Bo Patterson, the mayor of Port St. Joe, just south of Mexico Beach, rode out the storm in his house seven blocks from the beach, describing the scene outside as "very, very scary."
FEMA head Brock Long acknowledged that evacuation efforts in the area were slow compared with how fast the hurricane intensified. Michael grew from a tropical storm into a Category 4 hurricane in about 40 hours.
With a low barometric pressure recorded at 919 millibars, the measure of a hurricane's force, Michael ranked as the third-strongest storm on record to make landfall in the continental United States. Only Hurricane Camille on the Mississippi Gulf Coast in 1969 and the so-called Labor Day hurricane of 1935 in the Florida Keys were more intense.
US President Donald Trump declared a state of emergency for all of Florida, freeing up federal assistance to supplement state and local disaster responses.
About 3,500 Florida National Guard troops were deployed to assist with evacuations and storm recovery, along with more than 1,000 search-and-rescue personnel, Governor Rick Scott said.
The Pentagon said it had pre-positioned more than 2,200 active-duty military personnel, along with helicopters, high-water vehicles and swift-water boats for deployment as needed.
Even before landfall, the hurricane disrupted energy operations in the Gulf, cutting crude oil production by more than 40 per cent and natural gas output by nearly a third as offshore platforms were evacuated ahead of the storm's arrival.