Hurricane Irma a Category 4 storm as it heads for eastern Caribbean
Hurricane Irma has grown into a powerful Category 4 storm as it approaches the north-eastern Caribbean and is forecast to begin buffeting the region on Tuesday.
The storm had maximum sustained winds of 130mph and the US National Hurricane Centre said additional strengthening was expected.
Emergency officials warned the storm could dump up to 10in of rain, unleash landslides and dangerous flash floods and generate waves of up to 23ft.
"We're looking at Irma as a very significant event," said Ronald Jackson, executive director of the Caribbean Disaster Emergency Management Agency.
"I can't recall a tropical cone developing that rapidly into a major hurricane prior to arriving in the central Caribbean."
The storm's centre is forecast to move near or over the northern Leeward Islands late on Tuesday and early on Wednesday, the hurricane centre said.
US residents were urged to monitor the storm's progress in case it should turn northwards towards Florida, Georgia or the Carolinas.
"This hurricane has the potential to be a major event for the East Coast. It also has the potential to significantly strain Fema (Federal Emergency Management Agency) and other governmental resources occurring so quickly on the heels of (Hurricane) Harvey," said Evan Myers, chief operating officer of AccuWeather.
In the Caribbean, the governor of the British Virgin Islands urged people on Anegada island to leave if they could, noting that Irma's eye is expected to pass 35 miles from the capital of Road Town.
Antigua and Anguilla closed schools on Monday, and government office closures were expected to follow.
Puerto Rico and the US Virgin Islands expect 4in to 8in of rain and winds of 40-50mph with gusts of up to 60mph.
Puerto Rico governor Ricardo Rossello activated the National Guard, cancelled classes for Tuesday and declared a half-day of work.
He also warned of flooding and power outages. "It's no secret that the infrastructure of the Puerto Rico Power Authority is deteriorated," Mr Rossello said.
Meteorologist Roberto Garcia warned that Puerto Rico could experience hurricane-like conditions in the next 48 hours should the storm's path shift.
"Any deviation, which is still possible, could bring even more severe conditions to Puerto Rico and the US Virgin Islands," Mr Garcia said.
The US Virgin Islands said the school year would open on Friday instead of Tuesday.
Governor Kenneth Mapp said most hotels in the US territory were at capacity with 5,000 tourists. He noted the storm was expected to pass 40 miles north of St. Thomas and warned that the island could experience sustained winds as high as 80mph
"It's not a lot of distance," he said, adding: "It could affect us in a tremendous way. I'm not saying that to alarm anyone or scare anyone, but I want the Virgin Islands to be prepared."
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.
A hurricane watch was in effect for Puerto Rico, Vieques, Culebra, the British and US Virgin islands and Guadeloupe.
A tropical storm warning was in effect for Guadeloupe and a tropical storm watch for Dominica.