The third-strongest storm in recorded history to hit the mainland US has battered north-west Florida, flooding beach towns and snapping trees.
Rescue services are waiting for daylight to assess the full impact of Hurricane Michael, which made landfall on Wednesday afternoon as a category four storm with 155mph (250km/h) winds.
Two people, including a child, were killed by falling debris.
Having weakened to a tropical storm, Michael is on its way to the Carolinas.
Storm-surge warnings are in place between Panama City Beach and Keaton Beach in Florida, and between Ocracoke Inlet and Duck in North Carolina, the US National Hurricane Center says.
There are fears for people who ignored evacuation warnings in some of the areas now flooded.
Hundreds of thousands of homes and businesses were left without electricity in Florida, Alabama and Georgia.
Michael made landfall near Mexico Beach, Florida, at around 14:00 (18:00 GMT) on Wednesday.
Only the unnamed Labor Day hurricane, which hit Florida in 1935, and Hurricane Camille, which struck Mississippi in 1969, made landfall with greater intensity.
The Labor Day storm's barometric pressure (the lower the number, the stronger the storm) was 892 millibars and Camille's was 900, while Michael blew in with 919.
Michael was so powerful as it swept into Florida that it remained a hurricane as it moved further inland.
Its rapid intensification caught many by surprise, although the storm later weakened.
Unusually warm waters in the Gulf of Mexico turbo-charged the storm from a tropical depression on Sunday.
Only on Tuesday it was a category two hurricane but by Wednesday morning it had reached borderline category five, the highest level.
Florida officials say a man was killed when he was crushed in an incident involving a tree in Gadsden County.
In Seminole County, Georgia, a metal car-shelter lifted by a gust of wind hit a mobile home, killing a girl of 11.
Travis Brooks, director of Seminole County's emergency management agency, told ABC News there was "complete and total devastation".
The entire county was "pitch black" and there were no clear roads, he said.
Michael earlier reportedly killed at least 13 people as it passed through Central America: six in Honduras, four in Nicaragua and three in El Salvador.
More than 370,000 people in Florida were ordered to evacuate but officials believe many ignored the warning.
The coastal city of Apalachicola reported a storm surge of nearly 8ft (2.5m).
"There are so many downed power lines and trees that it's almost impossible to get through the city," local mayor Van Johnson was quoted as saying by Reuters news agency.
Images from Mexico Beach show many homes submerged in water, and there was severe damage to buildings in the state's Panama City area.
"We are catching some hell," Timothy Thomas, who rode out the storm with his wife in their home in Panama City, told the Associated Press news agency.
The storm knocked out power to a quarter of a million homes and businesses, as power lines were smashed by falling trees.
Federal Emergency Management Agency Director Brock Long said at the White House that he was especially concerned about buildings constructed before 2001, and not able to withstand such high winds.
"We just hope those structures can hold up," President Donald Trump responded. "And if not, that they're not in those structures."States of emergency have been declared in all or parts of Florida, Alabama, Georgia and North Carolina.
Schools and state offices in the area are to remain shut this week and Florida has activated 3,500 National Guard troops.
As of early Thursday morning local time, the storm winds had dropped to 60mph, the NHC said.
It warned that communities in north-west Florida and North Carolina faced the threat of life-threatening flooding as rising water moved inland from the coast.
The Carolinas are still recovering from the floods of Hurricane Florence.
Uni XC-SNU 1745
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