U.S. authorities sent an aircraft carrier and other navy ships to the Florida Keys to help with search and rescue operations Monday as a flyover of the hurricane-battered islands yielded what the governor said were scenes of devastation.

"My heart goes out to the people in the Keys," Gov. Rick Scott said. He says there was "devastation" and he hopes everyone who stayed behind survived Irma.

Scott said boats were cast ashore, water, sewer and power were knocked out, and "I don't think I saw one trailer park where almost everything wasn't overturned."

The scale of the damage inflicted by Irma began to come into focus as the hurricane weakened to a tropical storm and finally pushed out of Florida, but not before dealing a parting shot by triggering severe flooding around Jacksonville in the state's northeastern corner.

Around midday, Irma spread misery into Georgia and South Carolina as it moved inland with winds at 97 km/h.

Hurricane Irma slams southern Florida2:32

During its rainy, windy run up the full 640-kilometre length of Florida, Irma swamped homes, uprooted trees, flooded streets, snapped miles of power lines and toppled construction cranes.

"How are we going to survive from here?" asked Gwen Bush, who waded through thigh-deep floodwater outside her central Florida home to reach National Guard rescuers and get a ride to a shelter. "What's going to happen now? I just don't know."

More than 6.5 million homes and businesses statewide remained without power, and 180,000 people huddled in shelters. Officials warned it could take weeks for electricity to be restored to everyone.

Five deaths in Florida have been blamed on Irma, along with three in Georgia and one in South Carolina. At least 36 people were left dead in the storm's wake across the Caribbean.

Hurricane Irma

A man died when he crashed his pickup into a tree in the Florida Keys during Hurricane Irma. (Monroe County Sheriff's Department/Reuters)

Statewide, an estimated 13 million people, or two-thirds of Florida's population, remained without power. That's more than the population of New York and Los Angeles combined. Officials warned it could take weeks for electricity to be restored to everyone.

Irma was at one point the most powerful hurricane ever recorded in the open Atlantic, with winds up to 298 km/h. By Monday afternoon, its winds were down to 97 km/h.

The hurricane's wrath in the Sunshine State extended the full length of the state and reached from the west coast to the east.

The Keys felt Irma's full fury when it came ashore as a Category 4 storm with 209 km/h winds.

'Get out now'

Emergency managers there declared "the Keys are not open for business" and warned that there was no fuel, electricity, running water or cell service and that supplies were low and anxiety high.

"Help is on the way," they promised on Facebook.

In the Jacksonville area, close to the Georgia line, the storm surge brought some of the worst flooding ever seen there, with at least 46 people pulled from swamped homes.

The Jacksonville Sheriff's Office warned residents along the St. Johns River to "Get out NOW."

Hurricane Irma

Evacuees leave the Germain Arena, which was used as an evacuation shelter in Estero, Fla., on Monday. Hurricane Irma was downgraded to a tropical storm on Monday morning, but is still a serious threat with high winds and flooding. (Gerald Herbert/Associated Press)

"If you need to get out, put a white flag in front of your house. A T-shirt, anything white," the office said on its Facebook page. "Search and rescue teams are ready to deploy."

As Irma began moving into Georgia, a tornado spun off by the storm was reported on the coast, and firefighters inland had to rescue several people after trees fell on their homes.

A tropical storm warning was issued for the first time ever in Atlanta, and school was cancelled in communities around the state. More than 100,000 customers were without power in Georgia and over 80,000 in South Carolina.

Over the next two days, Irma is expected to push to the northwest, into Alabama, Mississippi and Tennessee.

Tropical Storm Irma tracking Monday 1:30 p.m.

People in the heavily populated Tampa-St. Petersburg area were braced for their first direct hit from a major hurricane since 1921. But by the time it struck in the middle of the night Monday, its winds were down to 160 km/h or less, and the damage was nowhere near as bad as expected.

In Redington Shores west of Tampa, Carl Roberts spent a sleepless night riding out Irma in his 17th-floor beachfront condo. After losing power late Sunday, he made it through the worst of the storm shaken but unhurt.

"The hurricane winds lashed the shutters violently, throughout the night," he wrote in a text message, "making sleep impossible."

As morning broke, he couldn't open the electric shutters to see outside.

More than 120 homes were being evacuated early Monday just outside Orlando as floodwaters started to pour in. Firefighters and National Guardsmen went door to door and used boats to ferry families to safety.

A few miles away, a huge sinkhole opened at the edge of an apartment building, swallowing air-conditioning units and bushes. Firefighters helped more than two dozen tenants relocate in the pounding rain and wind.‚Äč

3 storm deaths in Georgia

Tropical storm Irma claimed at least three lives in Georgia.

The Forsyth County Sheriff's Office said on its website Monday that a woman died from injuries she suffered when a tree fell on a vehicle in a private driveway.

The sheriff's office says deputies and firefighters tried to rescue the woman, but she died from her injuries. The sheriff's office said it was withholding the woman's name until her family and friends had been notified.

The storm is also being blamed for the death of a man in his 50s who died when a tree fell on his house just north of Atlanta and for the death of a 62-year-old man in rural southwest Georgia who had a heart attack after he climbed onto a shed Monday in a county where sustained winds exceeded 65 km/h.

Hurricane Irma Georgia

A City of Tybee police officer is seen on Tybee Island, Ga., on Monday. (Stephen B. Morton/Associated Press)

With tropical storm winds extending more than 640 kilometres from its centre, Irma has caused damage across the state from trees falling on inland homes to flooding in neighbourhoods on the Georgia coast.

About 800 flights were cancelled at Hartsfield-Jackson Atlanta International Airport, which remained operational Monday as its staff monitored storm conditions with help from the Federal Aviation Administration, airport spokesperson Andrew Gobeil said.

By Monday afternoon, more than 800,000 homes mostly in coastal and south Georgia were without power.

More deaths confirmed in Caribbean

Irma once was the most powerful hurricane ever recorded in the open Atlantic, a Category 5 with a peak wind speed of 300 km/h.

For days, forecasters warned Irma was taking dead aim at Florida. Irma made landfall as a Category 4 hurricane Sunday morning at Cudjoe Key, not far from Key West. It then rounded Florida's southwestern corner and hugged the coast closely as it pushed north.

Before Irma reached Florida, it left a swath of destruction in the Caribbean, and the number of confirmed deaths continued to rise on Monday. 

Cuba Hurricane Irma

People move through flooded streets in Havana on Sunday, after Hurricane Irma ripped through Cuba. On Monday, Cuban state media said 10 people were confirmed dead. (Ramon Espinosa/Associated Press)

Cuban state news media said 10 people died across the island as it was being battered by the then hurricane.

Most of them died in Havana, where chest-deep seawater pushed several blocks into densely populated neighbourhoods.

The state media said several of the deaths occurred in partial building collapses. Much of Cuba's housing stock is deteriorating.