Hurricane Irma is so record-breakingly strong it is impossible to hype and holds about twice the energy of all the bombs used in World War II, experts have said.
Louis Uccellini, director of the National Weather Service, said he was concerned about Florida up the east coast to North Carolina, starting with the Florida Keys.
He warned "all the hazards associated with this storm" are going to be dangerous.
Hurricane expert Kerry Emanuel of MIT calculated Irma holds about 7 trillion watts - twice the energy of all the bombs used in World War II.
So far, the storm has killed at least eight people and left 23 injured on French island territories, France's interior minister has said.
"This is not, by far, a definitive number... we have not explored all the parts of the island," Guadeloupe prefect Eric Maire said, adding the death toll was likely to rise in the next few hours.
Hurricane Irma had previously been described as a "potentially catastrophic" storm placed in Category 5, the highest US classification for hurricanes.
The storm blacked out much of Puerto Rico as it raked the US territory with heavy wind and rain while staying just out to sea, as it headed toward the Dominican Republic and Haiti.
To the east, authorities struggled to get aid to small Caribbean islands devastated by the storm's record 185mph (298kph) winds, while people in Florida rushed to get ready for a possible direct hit on the Miami area.
Nearly every building on Barbuda was damaged when the hurricane's core crossed almost directly over the island early Wednesday and about 60 percent of its roughly 1,400 residents were left homeless, Antigua and Barbuda Prime Minister Gaston Browne said.
President Donald Trump approved an emergency declaration for the US Virgin Islands and Puerto Rico, allowing the Federal Emergency Management Agency and other agencies to remove debris and give other services that will largely be paid for by the US government.
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