Hurricane Irma has grown into a dangerous Category 5 storm, the most powerful seen in the Atlantic in over a decade, as it roars toward islands in the northeast Caribbean on a path that could eventually take it to the United States.

The U.S. National Hurricane Center (NHC) said Irma is now the most powerful storm ever recorded in the Atlantic Ocean. By Tuesday afternoon, there were recorded winds of up to 297 km/h as it approached the Leeward Islands in the northwest Caribbean. 

As Irma approached the twin-island nation of Antigua and Barbuda, the hurricane centre said it was a "potentially catastrophic" storm. It was centred about 365 kilometres east of Antigua in the late morning and moving west at 22 km/h.

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.

If it stays on track and reaches the Florida Straits, the water there is warm enough that the already "intense" storm could become much worse with wind speeds potentially reaching 362 km/h, warned Kerry Emanuel, an MIT meteorology professor.

"People who are living there [the Florida Keys] or have property there are very scared, and they should be," Emanuel said.

Life-threatening storm surge

Irma's centre was expected to move over portions of the northern Leeward Islands late Tuesday and early Wednesday, the hurricane centre said, warning of "a life-threatening storm surge and large breaking waves" that could raise water levels from two to 3.5 metres above normal.

The storm's eye is expected to pass about 80 kilometres from Puerto Rico late Wednesday.

Empty shelves - Irma

A woman looks for candles in a supermarket in Pointe-a-Pitre, Guadeloupe, on Monday, trying to prepare for the arrival of Hurricane Irma. (Helene Valenzuela/AFP/Getty Images)

Irma is the strongest Atlantic hurricane since Rita in 2005, officials said.

"Puerto Rico has not seen a hurricane of this magnitude in almost 100 years," Carlos Anselmi, a U.S. National Weather Service meteorologist in San Juan, told The Associated Press.

Authorities warned that the storm could dump up to 31 centimetres of rain, cause landslides and flash floods and generate waves of up to seven metres. Government officials began evacuations and urged people to finalize all preparations as shelves emptied out across islands including Puerto Rico.

Life and death decisions

"The decisions that we make in the next couple of hours can make the difference between life and death," Puerto Rico Gov. Ricardo Rossello said. "This is an extremely dangerous storm."

Hurricane warnings were issued for 12 Caribbean island groups including Antigua, where buzzing chain saws and pounding hammers could be heard widely on Tuesday. Crews delivered water to neighbouring Barbuda, one of the islands closest to the hurricane's path.

Prime Minister Gaston Browne told the AP he was confident Barbuda would weather the storm because its shelter was built with reinforced concrete and equipped with a backup generator.

Hurricane Irma

People buy materials at a hardware store after Puerto Rico Gov. Ricardo Rossello declared a state of emergency in preparation for Hurricane Irma. (Alvin Baez/Reuters)

"I am satisfied that at a governmental level that we have done everything that is humanly possible to mitigate against the effects or the potential effects of this storm," he said. "What is really required now is for Antiguans and Barbudans ... to follow the warnings and to act appropriately so that we do not end up with any serious casualties or any fatalities."

Antigua's airport announced it was closing with an ominous statement advising visitors and residents to protect themselves from the "onslaught" of the storm: "May God protect us all."

Puerto Ricans braced for blackouts after the director of the island's power company told reporters 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.

Hurricane warnings and watches throughout region

Both Puerto Rico and the U.S. Virgin Islands expected 10-25 centimetres of rain and winds exceeding 60 km/h with gusts of up to 120 km/h.

"This is not an opportunity to go outside and try to have fun with a hurricane," U.S. Virgin Islands Gov. Kenneth Mapp warned. "It's not time to get on a surfboard."

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The National Hurricane Center shows the predicted travel path for Hurricane Irma. (National Hurricane Center)

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 U.S. and British Virgin islands.

On Tuesday afternoon, the Canadian government updated its travel advisories stating travel to those areas should be avoided. 

Hurricane watches were in effect for the Turks and Caicos, Guadeloupe and parts of the Bahamas, Haiti and the Dominican Republic.

In Florida, residents took advantage of the Labour Day holiday to empty many store shelves of drinking water and other supplies in advance of the storm.

Gov. 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 to duty Friday when the storm could be bearing down on the area. On Monday, Scott declared a state of emergency in all of Florida's 67 counties.

Scott will remove the toll on all of Florida's roadways as people try to evacuate areas that may be hit by Irma. The governor said tolls will remain suspended "for the duration of the storm's impacts to Florida."

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 2,420 kilometres east of the Lesser Antilles with maximum sustained winds of 65 km/h. It was moving west-northwest at 20 km/h and was expected to become a hurricane by Friday.

By Tuesday evening, U.S. President Donald Trump had declared emergencies in its territories in Irma's path. The declaration allows the Department of Homeland Security and the Federal Emergency Management Agency to co-ordinate disaster relief efforts in Florida, the U.S. Virgin Islands and Puerto Rico.