The dangerous Category 5 storm is wielding the most powerful winds ever recorded for a storm in the Atlantic Ocean. US President Donald Trump declared emergencies in Florida, Puerto Rico and the US Virgin Islands in preparation of Irma.
The most powerful Atlantic Ocean hurricane in recorded history made landfall on the smaller Leeward islands northeast Caribbean on Wednesday, following a path predicted to then rake Puerto Rico, the Dominican Republic, Haiti and Cuba before possibly heading for Florida over the weekend.
At the far northeastern edge of the Caribbean, authorities on the Leeward Islands of Antigua and Barbuda cut power and urged residents to shelter indoors as Hurricane Irma made its first contact with land on smaller islands.
Officials warned people to seek protection from Irma's "onslaught" in a statement that closed with: "May God protect us all."
The Category 5 storm had maximum sustained winds of 295 kilometres per hour (km/h) according to the US National Hurricane Centre in Miami.
"I hear it's a Cat 5 now and I'm terrified," Antigua resident Carol Joseph said as she finished her last trip to the supermarket before seeking shelter.
"I had to come back for more batteries because I don't know how long the current will be off."
Other islands in the path of the storm included the US and British Virgin Islands and Anguilla, a small, low-lying British island territory of about 15,000 people.
US President Donald Trump declared emergencies in Florida, Puerto Rico and the US Virgin Islands.
Warm water is fuel for hurricanes and Irma is over water that is 1 degree Celsius warmer than normal.
The 26 degree water that hurricanes need goes about 80 metres deep, said Jeff Masters, meteorology director of the private forecasting service Weather Underground.
Four other storms have had winds as strong in the overall Atlantic region but they were in the Caribbean Sea or the Gulf of Mexico, which are usually home to warmer waters that fuel cyclones.
Hurricane Allen hit 306 km/h (190 mph) in 1980, while 2005's Wilma, 1988's Gilbert and a 1935 great Florida Key storm all had 298 km/h (185 mph) winds.
The storm's eye was expected to pass about 80 km (50 miles) from Puerto Rico late Wednesday.
Hurricane-force winds extended outward up to 95 km (60 miles) from Irma's centre and tropical storm-force winds extended outward up to 280 km (175 miles).
The Northern Leeward Islands were expected to see waves as high as 3.3 metres (11 feet), while the Turks and Caicos Islands and southeastern Bahamas could see towering 6-metre (20-foot) waves later in the week, forecasters said.
"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."
The National Weather Service said Puerto Rico had not seen a hurricane of Irma's magnitude since Hurricane San Felipe in 1928, which killed a total of 2,748 people in Guadeloupe, Puerto Rico and Florida.
Expecting serious destruction of infrastructure
"The dangerousness of this event is like nothing we've ever seen," Puerto Rico Govenor Ricardo Rossello said.
"A lot of infrastructure won't be able to withstand this kind of force."
The director of the island's power company has warned that storm damage could leave some areas without electricity for about a week and other, unspecified areas for four to six months.
The utility's infrastructure has deteriorated greatly during a decade-long recession, and Puerto Ricans experienced an island-wide outage last year.
Government officials began evacuations and urged people to finalise all preparations as store shelves emptied out around Puerto Rico.
"The decisions that we make in the next couple of hours can make the difference between life and death," Rossello said. "This is an extremely dangerous storm."
No directly storm-related deaths were reported by Tuesday evening but a 75-year-old man died in the central Puerto Rico mountain town of Orocovis after he fell from a ladder while preparing for the hurricane, police said.
The eye of the storm was expected to roar westward on a path taking it north of millions of people in Puerto Rico, the Dominican Republic, Haiti and Cuba, but meteorologists warned that it could still cause life-threating storm surges, rains and mudslides.
The northern parts of the Dominican Republic and Haiti could see 25 cm (10 in) of rain, with as much as 51 cm (20 in) in the southeast Bahamas and Turks and Caicos.
Approaching the US
The storm seemed almost certain to hit the United States by early next week.
"You'd be hard pressed to find any model that doesn't have some impact on Florida." said University of Miami senior hurricane researcher Brian McNoldy.
In Florida, people also stocked up on drinking water and other supplies.
Governor Rick Scott activated 100 members of the Florida National Guard to be deployed across the state, and 7,000 National Guard members were to report for duty Friday when the storm could be approaching the area.
On Monday, Scott declared a state of emergency in all of Florida's 67 counties.
Officials in the Florida Keys geared up to get tourists and residents out of Irma's path, and the mayor of Miami-Dade county said people should be prepared to evacuate Miami Beach and most of the county's coastal areas.
Mayor Carlos Gimenez said the voluntary evacuations could begin as soon as Wednesday evening. He activated the emergency operation center and urged residents to have three days' worth of food and water.
New tropical storm Jose
A new tropical storm also formed in the Atlantic on Tuesday, to the east of Irma. The hurricane centre said Tropical Storm Jose was about 2,255 km (1,400 miles) east of the Lesser Antilles with maximum sustained winds of 75 km/h (45 mph). It was moving west-northwest at 19 km/h (12 mph) and was expected to become a hurricane by Friday.
Meanwhile, a tropical depression formed in the southwestern Gulf of Mexico off the coast of Mexico. The hurricane centre said the system could become a tropical storm while meandering in the Gulf for several days before making landfall in Mexico on Saturday.