Computer model projects have shifted west, placing Florida in the storm's crosshairs.
Forecasters said Monday there was an increasing chance that Hurricane Irma could bring its forecast Category 4 winds and rain to Florida.
In its 11 a.m. statement the National Hurricane Center warned of "impacts" from Irma on the Florida Peninsula and Keys.
"There is an increasing chance of seeing some impacts from Irma in the Florida Peninsula and the Florida Keys later this week and this weekend. In addition, rough surf and dangerous marine conditions will begin to affect the southeastern U.S. coast by later this week," the center noted in its forecast discussion. It added that it was still too early to tell what direct impacts Irma might have on the United States.
"However, everyone in hurricane-prone areas should ensure that they have their hurricane plan in place, as we are now near the peak of the season," the center said.
At 11 a.m. Irma was 650 miles east of the Leeward Islands. It was moving west-southwest at 14 mph with winds as high as 120 mph and a central pressure of 944 millibars.
The Hurricane Center's projected course for Irma had it striking the island of Cuba on Saturday as a major hurricane.
Meanwhile, Workers in the northeast Caribbean cleared drains and pruned trees as authorities urged islanders to prepare for Irma, a Category 3 storm that grew stronger Monday and was forecast to begin buffeting the region the following day.
Emergency officials warned that Irma could dump up to 10 inches (25 centimeters) of rain, unleash landslides and dangerous flash floods and generate waves of up to 23 feet (7 meters) as the storm draws closer.
A hurricane warning was issued for Antigua and Barbuda, Anguilla, Montserrat, St. Kitts and Nevis, St. Martin, Saba, St. Eustatius, St. Maarten and St. Barts.
"We're looking at Irma as a very significant event," Ronald Jackson, executive director of the Caribbean Disaster Emergency Management Agency, said by phone. "I can't recall a tropical cone developing that rapidly into a major hurricane prior to arriving in the central Caribbean."
A hurricane watch was in effect for Puerto Rico, Vieques, Culebra, the British and U.S. Virgin islands and Guadeloupe. A tropical storm watch was issued for Dominica.
Antigua and Anguilla shuttered schools on Monday, and government office closures were expected to follow.
On the tiny island of Barbuda, hotel manager Andrea Christian closed down the Palm Tree Guest House as Irma approached. She said she was not afraid even though it would be her first time facing a storm of that magnitude.
"We can't do anything about it," Christian said by phone, adding that she had stocked up on food and water. "We just have to wait it out."
The U.S. National Hurricane Center said islands farther north including the British and U.S. Virgin Islands and Puerto Rico should monitor the storm's progress.
Antigua Prime Minister Gaston Browne urged preventative measures such as cleaning drains and securing objects that could be sent flying by high winds. Workers began pruning branches that could potentially tear down utility lines.
"The passage of a hurricane is not a matter to be taken lightly, but we must not panic," Browne said in a statement.
The U.S. hurricane center said Irma had maximum sustained winds of 120 mph (195 kph) Monday morning, and some strengthening was expected through Tuesday night. The storm was centered about 560 miles (905 kilometers) east of the Leeward Islands and moving west-southwest at 14 mph (22 kph).
Puerto Rico Gov. Ricardo Rossello said 4 inches to 8 inches (10-20 centimeters) of rain were expected, as well as winds of 40-50 mph with gusts of up to 60 mph. He warned of flooding and power outages.
"It's no secret that the infrastructure of the Puerto Rico Power Authority is deteriorated," Rossello said. He activated the National Guard, canceled classes for Tuesday and declared a half-day of work.
Meteorologist Roberto Garcia warned that Puerto Rico could experience hurricane-like conditions in the next 48 hours.
"Any deviation, which is still possible, could bring even more severe conditions to Puerto Rico and the U.S. Virgin Islands," Garcia said.
In the Dominican Republic, Public Works Minister Gonzalo Castillo said workers there were also clearing away roadworks and cleaning out sewer drains and that President Danilo Medina would meet with emergency agencies.
Associated Press writer Ezequiel Abiu Lopez in Santo Domingo, Dominican Republic, contributed to this report.