As Hurricane Irma barrels across the Caribbean toward Florida, Miami Beach residents who refused to join the millions fleeing their state are out walking dogs and surfing in a ghostly city.

Need to walk the dog

One homeless man who sat on a bench in a promenade that ends at the sea showed no fear, and was unwilling to evacuate.

"I know the town, I live in South Beach. It's my island," said Phillip, 39, who only gave his first name.

"I'm not leaving it. I'm just gonna go where it's not getting flooded," he added, pointing to the bench where he was sitting.

Scott Abraham, a real estate agent in his early 40s, had a better plan:

wait out the storm on the fifth floor of his seven-floor beachfront apartment with his wife and two kids.

"My wife hates it when I say that, because it looks like we don't take care of the children", he said.

"If I lived in a house I would have left, but if it gets flooded here it's going to take a week at least to come back. I don't want that.

"My only problem," said Abraham, "is my puppy. He won't be able to do his walks."

The storm's forecast track has shifted slightly, so it now looks bound for Florida's west Gulf coast instead of the Atlantic coast.

But Irma is so wide that authorities were bracing for destructive storm surges on both coasts and the Florida Keys, the chain of low-lying islands that stretch south of Miami toward Cuba.

Curfews and blackouts

Cities across south Florida issued curfews starting Saturday evening, including Miami Beach, Miami and all of Broward County, home to the city of Fort Lauderdale.

Irma's blast through Cuba's coastline weakened the storm to a Category Three, but it still had dangerous winds and was expected to regain power before hitting the Florida Keys early Sunday, US forecasters said.

"The storm is here," said Florida Governor Rick Scott.

"The storm surge comes after the wind. Do not think the storm is over when the wind slows down."

Some 54,000 people have already taken refuge in the 320 shelters across the state -- and the number of evacuees continues to grow, Scott said.

Scott activated the Florida National Guard, nearly 30,000 soldiers along with 4,000 trucks, 100 helicopters and air evacuation gear to rescue stranded people after the storm.

By Saturday afternoon, blackouts were already being reported.

"We urge our customers to prepare for extended power outages, as approx. 4.1 million customers may lose power as a result of Irma," the Florida Power and Light utility company wrote on Twitter.

Scott also renewed his urgent plea for at least 1,000 volunteer nurses to help evacuees with special needs like the elderly.

Florida, a popular place for retirement, has a large population of elderly people. (AFP)