Hurricane Irma, one of the most powerful Atlantic storms in a century, has howled past Puerto Rico after thrashing several smaller Caribbean islands with tree-snapping winds, drenching rains and pounding surf on a collision course with Florida.

At least four people were reported killed on four different islands by Irma, which weather forecasters say was a "potentially catastrophic" Category 5 storm, the highest possible classification.

The dual-island nation of Antigua and Barbuda was especially hard hit.

The northernmost island, Barbuda, home to roughly 1800 people, was "totally demolished," with 90 per cent of all dwellings levelled, Prime Minister Gaston Browne said, according to island television broadcasts, with one person confirmed dead.

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A second storm-related fatality, that of a surfer, was reported on Barbados, and the French Government said at least two people were killed in Caribbean island territories of Saint Martin and Saint Barthelemy.

Irma, with top sustained winds of 300 km/h, was on track to reach Florida at the weekend, becoming the second major hurricane to hit the US mainland in as many weeks.

Irma's intensity could fluctuate and its precise course remained uncertain, but the storm is expected to remain at least a Category 4 before arriving in Florida.

Two other hurricanes formed today. Katia in the Gulf of Mexico posed no threat to the US but Hurricane Jose, about 1610km east of the Caribbean's Lesser Antilles islands, may threaten the US mainland.

This all comes after Hurricane Harvey claimed about 60 lives and caused an estimated US$180 billion in damages after slamming into Texas and Louisiana.

Cuba is also on hurricane alert with Havana residents queuing for food, water and petrol.

The eye of Irma passed just north of Puerto Rico, buffeting the US island territory's capital, San Juan, with heavy downpours and strong winds.

"The winds that we are experiencing right now are like nothing we have experienced before," Puerto Rico Governor Ricardo Rossello told CNN. "We expect a lot of damage, perhaps not as much as was seen in Barbuda."

At least half of Puerto Rico's homes and businesses lost electricity by nightfall, a utility executive posted on Twitter.

Aerial television footage of Barbuda showed a desolate, flooded landscape shorn of trees and foliage, littered with debris and overturned vehicles.

A high-end Barbuda property part owned by Hollywood star Robert De Niro was lost, De Niro's spokesman Stan Rosenfield said.

On its current path the core of Irma, which the NHC said marked 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, was expected to scrape the northern coast of the Dominican Republic and Haiti tomorrow.

It was on a track that would put it near the Turks and Caicos and southeastern Bahamas.

The heavy rains and wind of hurricane Irma cross through the northeastern part of the island in Fajardo, Puerto Rico. Photo / AP
The heavy rains and wind of hurricane Irma cross through the northeastern part of the island in Fajardo, Puerto Rico. Photo / AP

- Reuters, AAP