The small island of Barbuda is said to be “barely habitable”. Officials warn that St Martin is almost destroyed, and the death toll is likely to rise.
Irma, a category five hurricane, the highest possible level, is passing north of Puerto Rico.
Two other storms have strengthened to become hurricanes.
More than half of the island’s three million residents were without power as Irma caused heavy downpours and strong winds. Officials have said that power could be cut off for several days.
The most powerful Atlantic storm in a decade had wind speeds of 295km/h (185mph) and was expected to pass near or just north of the coast of the Dominican Republic on Thursday.
Hurricane Irma first hit the dual-island nation of Antigua and Barbuda. At least one death, of a child, was reported on Barbuda, where Prime Minister Gaston Browne said about 95% of the buildings had suffered some damage.
“It’s absolute devastation,” he said after flying over the island, home to some 1,600 people. “The island is literally under water. In fact, I’m of the view that, as it stands now, Barbuda is barely habitable.”
He told the BBC that 50% of the Barbuda population were now homeless and that it would cost $100m to rebuild the island.
However, Antigua, with a population of 80,000, escaped major damage, with no loss of life, he said earlier.
Officials have confirmed at least eight deaths and considerable damage in the French territories of St-Martin and Saint Barthélemy, popularly known as St Barts.
“It’s an enormous catastrophe – 95% of the island is destroyed,” top local official Daniel Gibbs was quoted as saying.
Significant damage was also reported in the Dutch section of St Martin, known as Sint-Maarten.
Sint-Maarten’s airport, the third largest in the Caribbean, has been destroyed.
The Dutch navy has sent two ships from nearby Aruba and Curacao to assist locals, according to media in the Netherlands.
US President Donald Trump said he and his aides were monitoring Irma’s progress. “But it looks like it could be something that will be not good. Believe me, not good,” he told reporters at the White House.
Projections suggest it could hit the state of Florida on Sunday.
Officials started evacuations of tourists and residents of Florida Keys, a resort archipelago.
Flights to and from several airports in Florida were being suspended, while Orlando’s international airport said that commercial flights would stop from 17:00 local time on Saturday.
A state of emergency had been declared for Florida, Puerto Rico and the US Virgin Islands, mobilising federal disaster relief efforts.
British Foreign Secretary Boris Johnson said the government was in touch with British overseas territories caught up in Irma, and was doing “everything we can to help those afflicted”.