Panicked residents stripped shelves bare in Miami Wednesday as they rushed to stockpile everything from bottled water to sandbags before Hurricane Irma pummels the sunshine state.

While motorists waited for hours to fill up at gas stations before driving northwards, there were similar lines at distribution centers as authorities handed out free sandbags.

"We're pretty scared as I'm a first-timer and don't know what we are coming up against," said singer Eugenia Sanchez as she loaded her ration of 10 sandbags into her car in Miami Beach.

"I think I'll write a song 'Irma, Please Leave Me,'" joked the 46-year-old who moved to Florida from Valencia in Spain.

"I've got to laugh or else I'll cry. In Miami Beach, anything that isn't boarded up might as well be a doll's house."

Florida is no stranger to hurricanes, but Irma's status as a Category Five storm has caused particular alarm ahead of its expected landfall on the state at some point over the weekend.

Irma, one of the most powerful Atlantic storms on record, has already caused death and destruction on a string of small Caribbean islands and is next projected to hit Puerto Rico and Cuba before ripping its way northwards towards Florida.

After dozens were killed by the flooding unleashed by Hurricane Harvey in Texas and Louisiana late last month, there are few illusions about the dangers posed by a monster storm.

The mayor of Miami, Carlos Gimenez, has so far decided not to order a mandatory evacuation for the city's coastal zones, including the popular tourist area of South Beach with its many bars, restaurants and nightclubs.

Gimenez said on Wednesday that there was still time to determine if a mass exodus was necessary or whether the storm loses some of its power or even veers away at the last moment.

- Battening boats -

Several gas stations along the main road from the Florida Keys to Miami were cordoned off with yellow tape, signaling they were out of gas, while lines of cars dozens deep gathered around open stations.

Nearly all local supermarkets had sold out of bottled water, with fights breaking out in some stores among shoppers trying to grab other essential supplies. 

In the Sea Isle Marina & Yacht Center, in downtown Miami, boat owners and workers were securing the boats with ropes, but there was no way to put them out of the water. 

"It's going to crash into Miami, right here. That's why I'm tying up the boats," said Cristian Gomez, the owner of three yachts he uses as tour boats.

"I am flying out of here tomorrow to Las Vegas. And I just have to rely on the insurance if it really smashes ashore right here," added the 30-year-old.

In a bid to minimize potential damage, crews of municipal workers could be seen scooping up debris and fallen tree branches that might otherwise smash into property.

So far, mandatory evacuation orders have been essentially limited to Monroe County on the southern tip of the state, which includes the Florida Keys.

But Florida Governor Rick Scott made clear on Wednesday that more evacuation orders could be issued in the days ahead and that there was no room for complacency.

"We're looking at all possible avenues to ensure everyone gets out," said Scott.

"If you're told to evacuate, don't wait. Get out quickly."