Hurricane Irma destroyed a quarter of homes on the Florida Keys, officials have said.

Evacuees have begun to return to the storm-ravaged islands of Key Largo, Tavernier and Islamorada to find their homes ripped apart and their businesses coated in seaweed.

The death toll from Irma, previously ranked as one of the most powerful Atlantic storms on record and the second major hurricane to strike the US mainland this season, has climbed to more than five dozen.

Of those, 43 were killed in the Caribbean and at least 18 in the southeastern United States.

Twelve Irma-related deaths were confirmed by Florida emergency management officials, while authorities in Georgia and South Carolina each reported three deaths from the storm and its immediate aftermath.

Florida suffers coast-to-coast battering from 130mph Irma

Destruction was widespread in the Keys, a resort island chain stretching southwest from the tip of the Florida Peninsula into the Gulf of Mexico and connected by a single, narrow highway and a series of bridges along a route of nearly 100 miles (160km).

Federal Emergency Management Agency administrator Brock Long said preliminary estimates suggested 25 per cent of the homes in the Keys were destroyed and 65 per cent sustained major damage.

"Basically, every house in the Keys was impacted," he said.

Although the Keys are studded with mansions and beachfront resorts, about 13 per cent of the people live in poverty and could face further hardships as the cleanup begins.

"People who bag your groceries when you're on vacation — the bus drivers, hotel cleaners, cooks and dishwashers — they're already living beyond paycheck to paycheck," Stephanie Kaple, who runs an organisation helping the homeless in the Keys, told the Associated Press.

Hurricane Irma – devastation caused in pictures Show all 45 1 /45 Hurricane Irma – devastation caused in pictures Hurricane Irma – devastation caused in pictures Recently planted palm trees lie strewn across the road as Hurricane Irma passes by in Miami Beach, Fla. 10 September 2017. AP Hurricane Irma – devastation caused in pictures Damage outside the Mercure hotel in Marigot, on the Bay of Nettle, on the island of Saint-Martin AFP/Getty Images Hurricane Irma – devastation caused in pictures People pick up debris in Fajardo as Hurricane Irma howled past Puerto Rico after thrashing several smaller Caribbean islands Reuters/Alvin Baez Hurricane Irma – devastation caused in pictures Large waves produced by Hurricane Irma crash into the end of Anglins Fishing Pier in Fort Lauderdale, Florida. The category 4 hurricane made landfall in the United States in the Florida Keys at 9:10 a.m. after raking across the north coast of Cuba. 10 September 2017 Getty Images Hurricane Irma – devastation caused in pictures A Royal Air Force Puma has been delivered to the US Virgin Islands to assist with the humanitarian efforts post Hurricane Irma. The Puma will be delivering Humanitarian Aid and Disaster Relief in support of the Department for International Development. Royal Air Force logisticians from RAF Brize Norton have assisted with the delivery of military personnel and aid cargo to the Caribbean to support disaster relief in the wake of Hurricane Irma. RAF aircraft including, C-17 A400M and Voyager are supporting a Joint Task Force of RAF, Royal Marines, Army and RN personnel who are supporting the Department for International Development as it delivers aid to stricken Caribbean Islands. MoD Hurricane Irma – devastation caused in pictures Flamingos at Zoo Miami, are shown in a temporary enclosure in a hurricane resistant structure within the zoo, Saturday, 9 September 2017 in Miami. Though most animals will reman in their secure structures, the cheetahs and some birds will ride out the storm in temporary housing. AP Hurricane Irma – devastation caused in pictures Boats are seen at a marina in South Beach as Hurricane Irma arrives at south Florida, in Miami Beach, Florida, U.S. 10 September 2017 Reuters Hurricane Irma – devastation caused in pictures Storm clouds are seen over Fisher Island as Hurricane Irma approaches on 9 September 2017 in Miami Beach, Florida. Florida is in the path of the Hurricane which may come ashore at category 4 Getty Images Hurricane Irma – devastation caused in pictures Waves crash over a seawall at the mouth of the Miami River from Biscayne Bay, Fla., as Hurricane Irma passes by. 10 September 2017 AP Hurricane Irma – devastation caused in pictures Some of the damage on Saint Martin EPA/Gerben Van Es/Dutch Department of Defence Hurricane Irma – devastation caused in pictures The skyline is seen as the outerbands of Hurricane Irma start to reach Florida on 9 September 2017 in Miami, Florida. Florida is in the path of the Hurricane which may come ashore at category 4. Getty Images Hurricane Irma – devastation caused in pictures A tree toped by hurricane Irma is seen on a empty street in Remedios, Cuba, 9 September 2017. Hurricane Irma reached Cuba bringing winds between 160 and 190 kilometers per hour. The hurricane has hit the north coast of the island. EPA Hurricane Irma – devastation caused in pictures James Constantineau loads sands bags in his truck as he prepares for the approach of Hurricane Irma Saturday, 9 September 2017, in East Palatka, Fla. Gov. Rick Scott is urging anyone living in an evacuation zone in southwest Florida to leave by noon as the threat of Hurricane Irma has shifted west. AP Hurricane Irma – devastation caused in pictures The Fort Louis Marina in Marigot is seen on 8 September 2017 in Saint-Martin island, devastated by Hurricane Irma. AFP Hurricane Irma – devastation caused in pictures Destruction in Orient Bay on the island of Saint-Martin AFP/Getty Hurricane Irma – devastation caused in pictures The wreckage in Orient Bay on the island of Saint-Martin AFP/Getty Hurricane Irma – devastation caused in pictures View of the aftermath of Hurricane Irma on Saint Martin Reuters Hurricane Irma – devastation caused in pictures A mobile network tower snapped in two by the hurricane on the island of Barbuda ABS TV Hurricane Irma – devastation caused in pictures A house reduced to rubble on the island of Saint Barthelemy AFP/Getty Images Hurricane Irma – devastation caused in pictures General view of damage on Saint Martin Reuters Hurricane Irma – devastation caused in pictures A damaged Casino Royale on Saint Martin after the passage of Hurricane Irma Anna Mazur/AFP Hurricane Irma – devastation caused in pictures An aerial photograph taken and released by the Dutch department of Defense shows the damage of Hurricane Irma in Philipsburg, Sint Maarten, the Dutch section of the Caribbean Island Gerben Van Es/AFP Hurricane Irma – devastation caused in pictures Flooded houses in Gustavia on the island of Saint-Barthelemy Kevin Barrallon/AFP Hurricane Irma – devastation caused in pictures The damage on the island of Saint-Martin, a day after Hurricane Irma hit AFP/Getty Images Hurricane Irma – devastation caused in pictures A man carrying an umbrella is battered by the wind in Fajardo, Puerto Rico Reuters/Alvin Baez Hurricane Irma – devastation caused in pictures A lone police car on patrol during the passing of Hurricane Irma in Fajardo, Puerto Rico Jose Jimenez/Getty Images Hurricane Irma – devastation caused in pictures Haitian people walk through the wind and rain on a beach in Cap-Haitien on September 7 as Hurricane Irma approaches Hector Retamal/AFP/Getty Images Hurricane Irma – devastation caused in pictures A flooded street on the island of Saint Martin AFP/Getty Images Hurricane Irma – devastation caused in pictures A tree collapsed on a house in Saint Martin Hurricane Irma – devastation caused in pictures A hotel in Saint Martin is gutted by floodwater during the hurricane Guadeloupe 1ère Hurricane Irma – devastation caused in pictures Cars submerged in Saint Martin Rinsy Xieng Hurricane Irma – devastation caused in pictures Debris floats amongst the floodwater in Saint Martin @la1ere Hurricane Irma – devastation caused in pictures Household items float down the street in Gustavia, Saint-Barthélemy Carole Greaux Hurricane Irma – devastation caused in pictures The coast of Saint Martin is flooded as the hurricane hits the island Météo Express Hurricane Irma – devastation caused in pictures A whole street underwater in Saint Martin @la1ere Hurricane Irma – devastation caused in pictures A car crashes into the tree amongst the chaos in Saint Martin @Bondtehond Hurricane Irma – devastation caused in pictures A building on the Saint Martin seafront, destroyed by the hurricane @Bondtehond Hurricane Irma – devastation caused in pictures A mobile home overturned at Princess Juliana International Airport in Saint Martin @Bondtehond Hurricane Irma – devastation caused in pictures Palm trees bend in the wind in San Juan, Puerto Rico as Hurricane Irma slammed across islands in the northern Caribbean Reuters/Alvin Baez Hurricane Irma – devastation caused in pictures A woman runs in the rain as Hurricane Irma slammed into San Juan, Puerto Rico Reuters/Alvin Baez Hurricane Irma – devastation caused in pictures A picture taken on September 5, 2017 shows a view of the Baie Nettle beach in Marigot, with the wind blowing ahead of the arrival of Hurricane Irma AFP/Getty Images Hurricane Irma – devastation caused in pictures A man rides past a boarded up house as part of preparations ahead of the arrival of Hurricane Irma on September 5, 2017, in the French overseas island of Guadeloupe Helene Valenzuela/AFP Hurricane Irma – devastation caused in pictures Employees of the Mercure Hotel fill sand bags on the Baie Nettle beach in Marigot, as part of the preparations for the arrival of Hurricane Irma Lionel Chamoiseau/AFP Hurricane Irma – devastation caused in pictures People in line at Costco, as they find out the store has ran out of water on September 5, 2017 in North Miami Michele Eve Sandberg/AFP Hurricane Irma – devastation caused in pictures Night view of the city of Cap-Haitien, in the north of Haiti, 240 km from Port-au-Prince, on September 5, 2017 Hector Retamal/AFP

Glimpses of Irma's economic toll are beginning to emerge, with Florida saying 31 state agencies had already amassed nearly $250 million in preparation and recovery expenses.

The number of people without electricity in the steamy late-summer heat dropped to 9.5 million — just under half of Florida's population. Utility officials warned it could take 10 days or more for power to be fully restored.

About 110,000 people remained in shelters across the state.

Overturned trailer homes are shown in the aftermath of Hurricane Irma in the Florida Keys (Matt McClain/The Washington Post via AP)

Irma was downgraded to a tropical depression late on Monday as it drifted into Alabama and was likely to dissipate further on Tuesday evening, the National Hurricane Centre said.