Irma is expected to become the second powerful storm to thrash the U.S. mainland in as many weeks after devastating Hurricane Harvey, but its precise trajectory remained uncertain.
The eye of Irma, a Category 5 storm packing winds of 185 miles per hour (295 km per hour), passed over the island of Barbuda east of Puerto Rico, early on Wednesday, the U.S. National Hurricane Center (NHC) in Miami reported, possibly hitting Florida on Saturday.
"We are hunkered down and it is very windy ... the wind is a major threat," said Garfield Burford, the director of news at ABS TV and Radio on the island of Antigua, south of Barbuda. "So far, some roofs have been blown off."
Most people who were on Antigua and Barbuda were without power and about 1,000 people were spending the night in shelters in Antigua, according to Burford.
"It's very scary ... most of the islands are dark so it's a very, very frightening," he said.
The amount of damage and the number of casualties were not known early on Wednesday. A 75-year-old man died while preparing for the storm in Puerto Rico's central mountains, police said.
Several other Leeward Islands, including Anguilla, Montserrat, St. Kitts and Nevis, as well as for the U.S. and British Virgin Islands, Puerto Rico and the Dominican Republic were under a hurricane warning.
"Preparations to protect life and property should be rushed to completion," the Hurricane Center said, warning that Irma "will bring life-threatening wind, storm surge and rainfall hazards" to those islands.
Along the beachfront of Puerto Rico's capital, San Juan, work crews scrambled to cover windows with plywood and corrugated metal shutters along Avenida Ashford, a stretch of restaurants, hotels and six-story apartments.
"I am worried because this is the biggest storm we have seen here," said Jonathan Negron, 41, as he supervised workers boarding up his souvenir shop.
The NHC said Irma ranked as one of the five most powerful Atlantic hurricanes during the past 80 years and the strongest Atlantic basin storm ever outside the Caribbean Sea and Gulf of Mexico.
"This is something without precedent," Rossello told a news conference.
U.S. President Donald Trump approved emergency declarations for Florida, Puerto Rico and the U.S. Virgin Islands, mobilizing federal disaster relief efforts, the White House said.
Authorities in the Florida Keys called for a mandatory evacuation of visitors to start at sunrise on Wednesday, and public schools throughout South Florida were ordered closed, some as early as Wednesday.
Residents of low-lying areas in densely populated Miami-Dade County were urged to move to higher ground by Wednesday as a precaution against coastal storm surges, three days before Irma was expected to make landfall in Florida.
Several tiny islands in the resort-heavy eastern Caribbean were the first in harm's way.
Hurricane watches were in effect for Guadeloupe, Haiti, the Turks and Caicos Islands and the southeastern Bahamas.
Airlines canceled flights to the region, and American Airlines added three extra flights to Miami from San Juan, St. Kitts and St. Maarten.
Residents of Texas and Louisiana were still recovering from Harvey, which struck Texas as a Category 4 hurricane on Aug. 25. It dumped several feet of rain, destroying thousands of homes and businesses, killed an estimated 60 people and displaced more than 1 million.
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