Monster Hurricane Irma hammered the Caribbean island of St Martin today as it packed a potentially catastrophic mix of pounding winds and rain and surging surf that was expected to make landfall in Florida over the weekend

The eye of potentially catastrophic category 5 Hurricane Irma is now closing in on the Virgin Islands, the US National Hurricane Center said in its latest advisory today.

Hurricane Irma is about 225 km east of San Juan Puerto Rico, with maximum sustained winds of 295 km/h, the Miami-based weather forecaster said.

"The extremely dangerous core of Irma will move over portions of the Virgin Islands very soon, pass near or just north of Puerto Rico this afternoon or tonight," the NHC said.

Irma will pass near or just north of the coast of the Dominican Republic Thursday, and be near the Turks and Caicos and the southeastern Bahamas late Thursday, it added.

Monster Hurricane Irma slammed across islands in the northern Caribbean today, packing a potentially catastrophic mix of pounding winds, raging surf and rain en route to a possible Florida landfall at the weekend.

The 295 km/h winds are expected to cause serious damage.

In Puerto Rico's capital of San Juan, a handful of people stood on the wind-whipped shore of a beachfront park this morning to take a last look at the ocean before seeking shelter.

Most businesses were closed and streets were almost empty. Cars packed parking lots of stores that were still open, with shoppers stuffing ice and water into their trunks.

Puerto Rico's Governor Ricardo Rossello urged the island's 3.4 million residents to seek refuge in one of 460 hurricane shelters.

It was not immediately clear how much damage Irma had done as it swept west.

Emergency officials on Antigua and Barbuda reported three injuries but minimal damage, with some roofs blown off. Communications between the islands were cut off, officials said.

Several other Leeward Islands, including Anguilla, Montserrat, St. Kitts and Nevis, as well as the US and British Virgin Islands, and the Dominican Republic were under a hurricane warning.

In Paris, the French government said it had delivered water and food to two of its overseas territories, St Martin and St Barthelemy, and that emergency response teams would be sent once the storm had passed. Power was knocked out on both islands, according to prefecture officials on Guadeloupe.

French Interior Minister Gerard Collomb said at least four buildings were damaged, including the prefecture, a fire brigade barracks and a police building and that low-lying regions had been flooded.

"For now we're not aware of any deaths," Collomb told reporters in Paris.

Residents in Florida are stocking up on essentials.

French Overseas Territories Minister Annick Girardin said "there were was much to fear" for citizens who had not heeded calls to seek safety in more secure buildings.

Irma ranked as one of the five most powerful Atlantic hurricanes in the last 80 years and the strongest Atlantic storm recorded by the outside the Caribbean Sea and Gulf of Mexico, according to the NHC.

Authorities in the Florida Keys ordered a mandatory evacuation of visitors to start at sunrise today. Public schools throughout South Florida were ordered closed, some as early as today.

Residents of low-lying areas in densely populated Miami-Dade County were urged to move to higher ground.

Florida Governor Rick Scott said there would be more mandatory evacuations around the state as Irma approached and as surges were expected to reach 3 metres.

"We can rebuild your home, we can't rebuild your life," Scott said on ABC's "Good Morning America."

Irma could be worse than Hurricane Andrew, which devastated the state in 1992. It is expected to become the second powerful storm to thrash the US mainland in as many weeks. Hurricane Harvey killed more than 60 people and caused as much as $180 billion in damage after hitting Texas late last month.