One of the most powerful Atlantic storms on record, the rare Category Five hurricane churned westward off the northern coast of Puerto Rico early morning on a potential collision course with south Florida where at-risk areas were evacuated.
St Martin -- a pristine resort known for its vibrant nightlife -- suffered the full fury of the storm, with rescuers on the French side of the island saying at least eight people had died there and another 21 were injured.
"It's an enormous catastrophe. Ninety-five percent of the island is destroyed," top local official Daniel Gibbs said in a radio interview.
"I'm in shock. It's frightening."
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Guadeloupe prefect Eric Maire called the situation in St Martin "dramatic," saying the island -- which is divided between the Netherlands and France -- was without drinking water or electricity, and warning the death toll was almost certain to rise.
"Barbuda now is literally rubble," Browne said.
One person is known to have died on the island of 1,600 residents, apparently a child whose family was trying to get to safer ground.
And on the island of Barbados, a 16-year-old professional surfer named Zander Venezia died while trying to ride a monster wave generated by the storm, the World Surf League said.
Irma was packing maximum sustained winds of up 295 kilometers per hour as it followed a projected path that would see it hit the northern edges of the Dominican Republic and Haiti on Thursday, continuing past eastern Cuba before veering north towards Florida.