SAN JUAN, Puerto Rico (REUTERS) - Hurricane Irma, one of the most powerful Atlantic storms in a century, killed eight people and plunged Puerto Rico into darkness on Thursday (Sept 7) as it swept through Caribbean islands while aiming for Florida.

The dead were reported on four islands. Weather forecasters have described Irma as a "potentially catastrophic" category five storm, the highest US classification for hurricanes.

At least half of Puerto Rico's homes and businesses were without power early on Thursday, according to Twitter posts and a message posted by an island utility executive.

The dual-island nation of Antigua and Barbuda was especially hard hit. The northernmost island, Barbuda, home to roughly 1,800 people, was "totally demolished," with 90 per cent of all dwellings there levelled, Prime Minister Gaston Browne said, according to island television broadcasts.

Browne said one person was killed on Barbuda. A second storm-related fatality, that of a surfer, was reported on Barbados.

The French government said at least six people were killed in Caribbean island territories of Saint Martin and Saint Barthelemy. Power was knocked out on both.

"This is not, by far, a definitive number .... We have not explored all the parts of the island," Guadeloupe prefect Eric Maire told reporters, adding that the death toll was likely to rise in the next few hours.

Early television footage of Saint Martin showed a devastated marina with boats tossed into piles, submerged streets and flooded homes.

"It is an enormous disaster, 95 per cent of the island is destroyed, I am in shock," Daniel Gibbs, chairman of a local council, told Radio Caribbean International.

Irma, with top sustained winds of 290 km per hour, was on track to reach Florida on Saturday or Sunday, becoming the second major hurricane to hit the US mainland in as many weeks.

On its current path, the core was expected to scrape the northern coast of the Dominican Republic and Haiti on Thursday. It was also on a track that would put it near the Turks and Caicos and south-eastern Bahamas by Thursday evening.

The NHC said it was the strongest hurricane ever recorded in the Atlantic Ocean and one of the five most forceful storms to hit the Atlantic basin in 82 years.

While Irma's intensity could fluctuate and its precise course remained uncertain, the storm was expected to remain at least a Category four before arriving in Florida.