The storm had sustained winds of up to 100km/h when it crossed into Georgia, the National Hurricane Centre said.

The body of a 62-year-old man who climbed a ladder behind his home was found under debris on the roof of his shed in southwest Georgia, Worth County sheriff’s spokeswoman Kannetha Clem said. His wife had called 911 saying he’d had a heart attack.

“He was lodged between two beams and had a little bit of debris on top of him,” Clem said. “He was on the roof at the height of the storm.”

Another man, in his 50s, was killed just outside Atlanta when a tree fell on his house, Sandy Springs police Sergeant Sam Worsham said.

And a woman died when a tree fell on a vehicle in a private driveway, according to the website of the Forsyth County Sheriff’s Office.

Charles Saxon, 57, became South Carolina’s first recorded death when he was struck by a tree limb while clearing debris outside his home in Calhoun Falls, according to a statement from Abbeville County Coroner Ronnie Ashley.

About 800 flights had been cancelled at Hartsfield-Jackson Atlanta International Airport, which remained operational Monday, even as many planes turned corners of the tarmac into a parking lot.

Earlier high winds snapped power lines and left almost 6.5 million homes and businesses without power in Florida, Georgia, South Carolina and Alabama, state officials and utilities said. They said it could take weeks to complete repairs.

Miami International Airport, one of the busiest in the country, halted passenger flights through at least Monday.

Police in Miami-Dade County said they had made 29 arrests for looting and burglary. Fort Lauderdale police said they had arrested 19 people for looting.

About two dozen vehicles filled with people hoping to return home after fleeing the Florida Keys, where Irma roared ashore on Sunday with sustained winds of up to 210km/h, lined up near the entrance to the highway that connects the archipelago to the mainland with a series of bridges and causeways.

They expressed anger at police who asked them to drive to a racetrack a few miles away to register first.

“This is how people are going to die – nobody’s going to want to leave the Keys,” shouted Shelby Bentley. “I’ve been in the Keys for 40 years. It’s the first time I’ve evacuated from a hurricane. It’ll be my last time.”

Officials in Monroe County, where the Keys are located, said most of the islands still lacked fuel, electricity, running water and cell service on Monday.

“Supplies are running low and anxiety is running high,” the county said in a statement posted online.

Irma hit Florida after powering through the Caribbean as a rare Category 5 hurricane. It killed 38 people, including 10 in Cuba, which was battered over the weekend by ferocious winds and 11-metre waves.

A week earlier Hurricane Harvey flooded a wide swathe of Houston. Nearly three months remain in the official Atlantic hurricane season.

Northeastern Florida cities including Jacksonville were flooding on Monday, with city sheriffs pulling residents from waist-deep water.

“Stay inside. Go up. Not out,” Jacksonville’s website warned residents. “There is flooding throughout the city.”

The storm did some US$20 billion to US$40 billion in damage to insured property as it tore through Florida, catastrophe modelling firm AIR Worldwide estimated.

That estimate, lower than earlier forecasts of up to US$50 billion in insured losses, helped spur a relief rally on Wall Street as fears eased that Irma would cut into US economic growth.

Some 6.5 million people, about one-third of Florida’s population, had been ordered to evacuate their homes ahead of Irma’s arrival. More than 200,000 people sought refuge in about 700 shelters, according to state data.

As shelters began to empty on Monday, some 7,000 people filed out of Germain Arena in Estero, south of Fort Myers. The crowd included Don Sciarretta, who rode out the storm with his 90-year-old friend, Elsie Johnston, who suffers from Alzheimer’s disease.

Sciarretta, 73, spent two days without sleep, holding up a slumped-over Johnston and making sure she did not fall out of her chair. He relied on other people in the shelter to bring the pair food, often after waiting on hours-long lines.

“For the next storm, I’ll go somewhere on my own like a hotel or a friend’s house,” Sciarretta said. “I’m not going through this again.”

Shelters across western Florida opened, filled up and often closed because of overcrowding after the storm made a western shift on Saturday. The jam-packed Germain Arena, a sea of cots and blankets, closed to new occupants on Saturday night.

US President Donald Trump, attending a ceremony at the Pentagon on the 16th anniversary of the September 11 attacks, vowed a full response to Irma as well as ongoing federal support for victims of Hurricane Harvey, which flooded Texas.

“These are storms of catastrophic severity and we are marshalling the full resources of the federal government to help our fellow Americans,” Trump said.

Florida’s largest city, Miami, was spared the brunt of the storm but still suffered heavy flooding. Boats, including one named “Lucky Duck,” had been driven ashore by the storm and utility crews were on the streets there clearing downed trees and power lines.

Additional reporting by Associated Press