Hurricane Irma has caused widespread destruction across the Caribbean, reducing buildings to rubble and leaving at least nine people dead.
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 Puerto Rico'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 sustained 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.