Irma now upgraded to a Category 5 hurricane which means sustained winds of 280 kph. Puerto Rico has declared a state of emergency.
Hurricane Irma strengthened into a dangerous Category 5 storm on Tuesday as it roared toward the northeast Caribbean on a path that could take it to the United States.
The US National Hurricane Center said Irma had sustained winds of 280 kph and was centred about 440 km east of Antigua. It was moving westwards at 22 kph.
The centre said there was a growing possibility that the storm's effects could be felt in Florida later this week and over the weekend, though it was still too early to be sure of its future track: "Everyone in hurricane-prone areas should ensure that they have their hurricane plan in place."
Irma's centre was expected to move near or over the northern Leeward Islands late on Tuesday and early Wednesday, the hurricane centre said. The eye was then expected to pass about 80 km (50 miles) from Puerto Rico late on Wednesday.
Authorities warned that the storm could dump up to 25 cm of rain, cause landslides and flash floods and generate waves of up to seven meters. Government officials began evacuations and urged people to finalize all preparations as shelves emptied out across the islands, including Puerto Rico.
"The decisions that we make in the next couple of hours can make the difference between life and death," Puerto Rico Governor Ricardo Rossello said. "This is an extremely dangerous storm."
Residents on the US East Coast were urged to monitor the storm's progress in case it should turn northward toward 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 and other governmental resources occurring so quickly on the heels of (Hurricane) Harvey," Evan Myers, chief operating officer of AccuWeather, said in a statement.
In the Caribbean, hurricane warnings were issued for 12 island groups, including the British Virgin Islands, where the governor urged people to evacuate the tiny island of Anegada if they could ahead of the storm. The island has a maximum elevation of 8.5 m above sea level.
Vivian Wheatley, proprietor of the Anegada Reef Hotel, planned to stay behind. She said she would stay in one of the hotel rooms and take advantage of the generator since there were no guests.
"We know it's a very powerful (storm), and we know it's going to be very close," she said. "Let's hope for the best."
People in the US territory of Puerto Rico braced for blackouts after the director of the island's power company predicted that storm damage could leave some areas without electricity for four to six months. But "some areas will have power (back) in less than a week," Ricardo Ramos told radio station Notiuno 630 AM.
“Despite the economic challenges Puerto Rico is facing, the approved budget has $15 million for the emergency fund,” Governor Ricardo Rossello said in a statement.
The utility's infrastructure has deteriorated greatly during a decade-long recession, and Puerto Ricans experienced an island-wide outage last year.
Both Puerto Rico and the US Virgin Islands expected 10-20 cm (4 inches to 8 inches) of rain and winds of 40-50 mph (64 to 80 kph) with gusts of up to 60 mph (96 kph).
The island of about 3.4 million people has 456 emergency shelters prepared to house up to 62,100 people.
To help residents prepare for the storm, the Puerto Rican government activated a price freeze on basic necessities, including food and water, medicines, power generators and batteries.
"This is not an opportunity to go outside and try to have fun with a hurricane," US Virgin Islands Governor Kenneth Mapp warned. "It's not time to get on a surfboard."
A hurricane warning was posted for Antigua and Barbuda, Anguilla, Montserrat, St Kitts and Nevis, St Martin, Saba, St Eustatius, St Maarten and St Barts, Puerto Rico, and the US and British Virgin islands. A tropical storm warning was in effect for Guadeloupe and Dominica.
Meanwhile in Florida, residents took advantage of the Labor Day holiday to empty many store shelves of drinking water and other supplies in advance of the storm. Also Monday, Florida Governor Rick Scott declared a state of emergency for the state's 67 counties to give local governments "ample time, resources and flexibility" to prepare.
American Airlines added extra flights out of two Caribbean islands to get people out of the path of Irma.
The flights were added in St Maarten, St Kitts and Nevis. Those are in addition to regularly scheduled flights on Tuesday and Wednesday to Miami.