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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 is a category five hurricane, the highest possible level.
It is currently north of the Dominican Republic, heading towards Turks and Caicos.
The low-lying Turks and Caicos islands, a British overseas territory, are said to be at risk of a storm surge, with the possibility of destructive waves up to 6m (20ft) higher than usual.
Virginia Clerveaux, director of the Turks and Caicos Department of Disaster Management and Emergencies, told the BBC that even inland areas could be inundated.
"We are now trying to remind them [the people of the islands] that this is a category 5, and in the history of the Turks and Caicos islands this is the largest storm we have ever been impacted or threatened by.
"So there is a need to ensure that we have maximum preparations in place... We have been saying to persons to ensure that they are prepared, ensure they can shelter safely, they have sufficient food and drinking water for two to three days."
Meanwhile, the head of the US emergency agency has said that Hurricane Irma will have a "truly devastating" impact when it hits southern coastal areas of the United States.
"The majority of people along the coast have never experienced a major hurricane like this," Federal Emergency Management Agency (Fema) chief Brock Long told CNN.
The most powerful Atlantic storm in a decade, Irma has sustained wind speeds of 285km/h (180mph).
(BBC)
