Hurricane Irma evacuees are returning to scenes of devastation in the Florida Keys with reports of a quarter of homes destroyed on the low-lying islands.
Evacuees from Hurricane Irma are returning to the Florida Keys, where sunrise will give them a first glimpse of devastation that has left countless homes and businesses in ruins.
Categorised as one of the most powerful Atlantic storms on record, Irma claimed more than 60 lives, officials said.
At least 18 people died in Florida and destruction was widespread in the Keys, where Irma made initial US landfall on Sunday to become the second major hurricane to strike the mainland this season.
A resort island chain that stretches from the tip of the state into the Gulf of Mexico, the Keys are connected by a bridges and causeways along a narrow route of 160km.
"I don't have a house. I don't have a job. I have nothing," said Mercedes Lopez, 50, whose family fled north from the Keys town of Marathon on Friday and rode out the storm at an Orlando hotel, only to learn their home was destroyed, along with the petrol station where she worked.
The Keys had been largely evacuated by the time Irma barrelled ashore as a Category 4 hurricane with sustained winds of up to 215km/h.
Initial damage assessments found 25 per cent of homes there were destroyed and 65 per cent suffered major damage, Federal Emergency Management Agency administrator Brock Long said.
Authorities allowed re-entry to the islands of Key Largo, Tavernier and Islamorada for residents and business owners on Tuesday. The extent of the devastation took many of the first returnees by surprise.
"I expected some fence lines to be down and some debris," said Orlando Morejon, 51, a trauma surgeon from Miami as he hacked away at a tree blocking his Islamorada driveway. "We were not expecting to find someone else's sailboat in our backyard."
A boil water notice was in effect for the Keys late on Tuesday, while its airports remained closed to commercial flights.
Several major airports in Florida that had halted passenger operations resumed with limited service on Tuesday, including Miami International, one of the busiest in the United States.
Across Florida and nearby states, some 5.8 million homes and businesses were late on Tuesday estimated to be still without power, down from a peak of 7.4 million on Monday.
Florida's largest utility, Florida Power & Light Co , said western parts of Florida might be without electricity until September 22.
The state's largest city, Jacksonville, in its northeastern corner, was still recovering from heavy flooding on Wednesday.
While damage across Florida was severe, it paled in comparison with devastation wrought by Irma in parts of the Caribbean, which accounted for the bulk of the hurricane's fatalities.
It destroyed about one-third of the buildings on the Dutch-governed portion of the eastern Caribbean island of St. Martin, the Dutch Red Cross said.
Irma was a post-tropical cyclone late on Tuesday as it drifted north as it brought rain to the Mississippi Valley, the National Hurricane Centre said.
It hit the US soon after Hurricane Harvey, which ploughed into Houston late last month, killing about 60 and causing some $US180 billion ($A224 billion) in damage, mostly from flooding.