A weakened but still dangerous Irma is pushing inland as it hammers Florida with winds and floods that have created hazards even for rescuers trying to help beleaguered residents.
Irma, once ranked as one of the most powerful hurricanes recorded in the Atlantic, came ashore in Florida as a category four storm on Sunday and battered towns as it worked its way up the state.
The storm gradually lost strength, weakening to a tropical storm on Monday morning as it moved towards southern Georgia, the National Hurricane Center said.
It is expected to weaken to a tropical depression by Tuesday afternoon.
Irma is centred about 170km north-northwest of Tampa and is moving north-northwest near 30km/h.
A large area of the state's east and west coasts remained vulnerable to storm surges, when hurricanes push ocean water dangerously over normal levels.
That risk extended to the coast of Georgia and parts of South Carolina.
Florida emergency management director Bryan Koon said officials would wait until first light on Monday to begin rescue efforts and assess damage.
Damage appeared to be severe in the Florida Keys, where Irma first came ashore as a category four hurricane with sustained winds up to 215km/h early on Sunday.
On Sunday, Irma claimed its first US fatality - a man found dead in a pick-up truck that had crashed into a tree in high winds in Marathon in the Florida Keys, local officials said.
The storm killed at least 28 people as it raged westward through the Caribbean on the way to Florida, devastating several small islands, and grazing Puerto Rico, the Dominican Republic and Haiti before pummelling parts of Cuba's north coast with 11m waves.
Irma was ranked a category five, the rare top end of the scale of hurricane intensity, for days, and carried maximum sustained winds of up to 295km/h when it crashed into the island of Barbuda on Wednesday.
Its ferocity as it bore down on hurricane-prone Florida prompted one of the largest evacuations in US history.
About 6.5 million people, about one-third of the state's population, had been ordered to evacuate southern Florida to shelters, hotels or relatives in safer areas.
High winds snapped power lines and left about 5.8 million Florida homes and businesses without power.
Many of the evacuation orders extended until at least Monday due in part to flooding, massive power outages and downed electric lines.
Miami International Airport, one of the busiest in the country, halted passenger flights through at least Monday.
Irma was forecast to weaken to a tropical storm as it moved near Florida's northwestern coast on Monday morning, the National Hurricane Center said.
It would cross the eastern Florida Panhandle and move into southern Georgia later in the day, dumping as much as 41cm of rain, it said.
