Hurricane Irma has begun its assault on Florida, with the storm’s northern eyewall reaching the lower Florida Keys as a powerful Category 4 storm.

Irma lashed the area with maximum sustained winds near 215 kph and the US National Hurricane Centre said it was expected to remain a powerful storm as it moved through the Florida Keys and near the state’s west coast.

As of 7:00am local time on Sunday (9pm AEST), the hurricane was centred about 25 kilometres southeast of Key West, Florida, and was moving northwest at 13 kph.

Tens of thousands of people huddling in shelters watched for updates as the storm swung to the west, now potentially sparing Tampa and Miami the catastrophic head-on blow forecasters had been warning about for days.

In the Tampa Bay area, access to all of Pinellas County’s barrier islands, including the popular spring break destination of Clearwater Beach, was shut off.

The leading edge of the immense storm bent palm trees and spit rain across South Florida, knocking out power to more than 430,000 homes and businesses, as the eye approached Key West.

As the hurricane’s eye approached the Florida Keys, Carol Walterson Stroud, 60, and her family were huddled in a third floor apartment at a senior centre in Key West.

“We are good so far,” she said in a text message just before 5:30 am. “It’s blowing hard.”

Ms Stroud said she planned to step outside once the eye of the hurricane passed over. She said she has stood in the eye of a hurricane before and it was “total peace and quiet”.

Florida Governor Rick Scott had warned residents in the state’s evacuation zones on Saturday that “this is your last chance to make a good decision”. About 6.4 million people were told to flee.

But because the storm is 560 to 640 miles wide, the entire Florida peninsula was exposed. Forecasters said the greater Miami area of 6 million people could still get life-threatening hurricane winds and storm surge of 1.2 to 1.8 metres.

Irma was a Category five storm, the highest ranking possible, when it crashed into Cuba on Saturday morning.

At least 25 people died when Irma earlier hit several Caribbean islands.

Irma is forecast to dump up to 50cm of rain over Florida and southeast Georgia through to Monday.

This would be less than the 127cm Hurricane Harvey dropped on parts of Texas and Louisiana two weeks ago, killing at least 60 people and causing an estimated $US180 billion ($A223 billion) in property damage.