Already dangerous with sustained winds of 135 mph (215 kph), Earl is expected to gain more strength before potentially brushing the U.S. East Coast this week and bringing deadly rip currents.
The U.S. National Hurricane Center in Miami warned coastal residents from North Carolina to Maine to watch the storm closely.
"Any small shift in the track could dramatically alter whether it makes landfall or whether it remains over the open ocean," said Wallace Hogsett, a meteorologist at the center. "I can't urge enough to just stay tuned."
In the Caribbean, Earl caused flooding in low-lying areas and damaged homes on islands including Antigua and Barbuda, Anguilla and St. Maarten. Several countries and territories reported power outages. Cruise ships were diverted and flights canceled across the region.
"We are getting a battering with wind and rain," said Martin Gussie, a police officer in Anguilla. Several utility poles were down and a couple of roofs had blown away, and it was still too dangerous to go out and assess the full extent of damage, he said.
The storm's center passed just north of the British Virgin Islands on Monday afternoon. It was gradually moving away from the Caribbean and was forecast to approach the U.S. Mid-Atlantic region around Thursday, before curving back out to sea, potentially swiping New England or far-eastern Canada.
The Hurricane Center said it was too early to say what effect Earl would have in the U.S., but warned it could at least kick up dangerous rip currents. A surfer died in Florida and a Maryland swimmer has been missing since Saturday in waves spawned by former Hurricane Danielle, which weakened to a tropical storm Monday far out in the north Atlantic.
Craig Fugate, administrator of the Federal Emergency Management Agency, said Earl's approach ought to serve as a reminder for Atlantic coastal states to update their evacuation plans.
"It wouldn't take much to have the storm come ashore somewhere on the coast," Fugate said. "The message is for everyone to pay attention."
The rapid development of Earl, which only became a hurricane Sunday, took some islanders and tourists by surprise.
Wind was already rattling the walls of Lila Elly Ali's wooden house in Anegada, the northernmost of the British Virgin Islands, when she and her son went out to nail the doors shut Monday.
"They say the eye of the storm is supposed to come close to us, so we've just got to pray. Everyone here is keeping in touch, listening to the radio," the 58-year-old said by phone from the island of 280 people.
At El Conquistador Resort in Fajardo, Puerto Rico, people lined up at the reception desk, the lights occasionally flickering, to check out and head to the airport. There, more delays awaited.
John and Linda Helton of Boulder, Colorado, opted to ride out the storm. The couple, celebrating their 41st wedding anniversary, finished a cruise Sunday and planned to spend three days in Puerto Rico.
"There was a huge line of people checking out as we were coming in, and I thought it was just that summer vacation must be over," said John Helton, a real estate appraiser. "But we paid for the room, so we might as well stick it out."
"I don't think we could get a flight even if we wanted to leave," Linda Helton added.
There were no reports so far of major damage from Earl.
In St. Maarten, sand and debris littered the streets, and winds knocked down trees and electricity poles and damaged roofs. But police spokesman Ricardo Henson said there was no extensive damage to property.
Alisha Daya, a 24-year-old tourist from Milwaukee, wore earplugs but still had trouble sleeping overnight because of the wind and crashing waves at the Oyster Bay Beach Resort.
"It was loud because we were right on the ocean," Daya said, adding that the storm delayed their planned departure Monday but the worst seemed to be over.
In Antigua, at least one home was destroyed but there were no reports of serious injuries. Governor General Dame Louise Agnetha Lake-Tack declared Monday a public holiday to keep islanders off the road and give them a chance to clean up.
Jeremy Collymoore, head of the Barbados-based Caribbean Disaster Emergency Response Agency, said islands such as Antigua and Anguilla appeared to have been spared worse damage because they were raked by the system's northwestern quadrant - the most forgiving part.
Mudslides and flooding were still a risk, with 4 to 8 inches (10 to 20 centimeters) of rain forecast to fall on islands including Puerto Rico.
The U.S. Virgin Islands imposed a curfew for Monday night.
The Hovensa LLC oil refinery in St. Croix said operations were normal except for the harbor, which was closed along with all ports in the U.S. Virgin Islands by order of the Coast Guard.
Monday afternoon, Earl was about 110 miles (180 kilometers) northeast of San Juan, Puerto Rico, and headed west-northwest at 15 mph (24 kph), according to the center in Miami. Hurricane-force winds extended outward up to 70 miles (110 kilometers) from its center.
Also Monday, Tropical Storm Fiona formed behind Earl in the Atlantic east of the Leeward Islands.
--- Associated Press writers Ben Fox in Fajardo, Puerto Rico, Anika Kentish in St. John's, Antigua, Judy Fitzpatrick in Philipsburg, St. Maarten, Clive Bacchus in Basseterre, St. Kitts, David McFadden in San Juan and Sofia Mannos in Washington contributed to this report.