Whether by plane, car or train, hundreds of thousands of people scrambled to get out of South Florida ahead of Hurricane Irma, a massive system forecasters say is the most powerful storm to hit the Atlantic Coast in more than a decade.

The storm, with maximum sustained winds near 295km/h, moved over the Leeward Islands before moving to the northern Virgin Islands and Puerto Rico.

Forecasters say it will reach south Florida sometime between Saturday and Monday.

To accommodate surging demand of people trying to flee, airlines including American and Delta added flights or brought in larger planes and waived change fees for passengers who need to cancel or rebook their flights.

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Even so, hundreds of flights were cancelled.

American Airlines announced it would begin winding down its operations in Florida and had cancelled flights at its Miami hub as well as to airports in Fort Lauderdale, Fort Myers, Sarasota and West Palm Beach. In addition, it cancelled a handful of international flights from Europe and South America that were scheduled to land in Miami on Saturday.

Many of those leaving said they didn't want to gamble on getting stranded.

Janet and Tom Wrabel of Fairfield, Connecticut, had been visiting their daughter in Fort Lauderdale when they saw the storm brewing and decided to cut their holiday a couple of days short.

"We would rather be here reclining by a pool right now instead of getting on a plane," said Tom, 62, as the couple prepared to leave.

Officials at Key West International Airport, said they would suspend commercial operations at the end of the day tomorrow, and Miami International Airport advised travellers it will halt operations at the airport when winds reach 88km/h. Generally airlines do not operate in sustained crosswinds that exceed 56km/h, and the Federal Aviation Administration tower ceases operations after winds of 88km/h, the airport noted.

On the state's roadways, long lines of cars and trucks could be seen travelling north on the Florida turnpike as tourists and residents fled the Florida Keys - among the first areas to be under a mandatory evacuation order. Service stations were also jammed as Floridians rushed to fill their tanks before Irma's arrival. Grocery stores reported running out of bottled water.

The main worry remained the storm's impact on South Florida, home to six million people. US President Donald Trump yesterday declared an emergency in the state as well as the US Virgin Islands and Puerto Rico. Florida Governor Rick Scott (R) has ordered all 7000 members of the Florida National Guard report for duty on Saturday.

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Evacuations began in the Florida Keys, and state transportation officials were "aggressively clearing evacuation routes," officials said.

The main routes out of south Florida are Interstate 95 and the Florida Turnpike, which can be prone to gridlock on the best of days. In an effort to ease congestion, the Governor ordered that no tolls be collected.

State officials say more mandatory evacuations would be ordered as the storm nears, and some urged residents to evacuate the area early and expect clogged roadways.