The Caribbean island of Barbuda has been lashed by the worst hurricane to ever make landfall in the area.
Warnings have been put out across other West Indian islands to prepare for the storm.
Meanwhile, evacuations of parts of Florida have commenced as experts upped the chances of it hitting the US state to as much as a 90 per cent probability.
The US National Hurricane Center warned:
Irma is a potentially catastrophic category five hurricane and will bring life-threatening wind, storm surge and rainfall hazards to portions of the northern Leeward Islands... Preparations should be rushed to completion.
JLT Re catastrophe experts called the latest hurricane "rare" adding: "Irma is likely to be the most intense hurricane to ever make landfall in the Caribbean, eclipsing David (1979) with 160 mph winds hitting Hispaniola."
Florida's Key West has been subject to mandatory evacuation.
"The most likely outcome remains a US landfall, with increased probability today at 85 per cent to 90 per cent (an increase from 75 to 80 per cent yesterday). Florida remains the most likely state for US landfall, expected early on Sunday," JLT Re said.
"We're emphatically telling people you must evacuate. You cannot afford to stay on an island with a category five hurricane coming at you," said Florida's Monroe County emergency operations centre director Martin Senterfitt.
The Prime Minister of the Bahamas has ordered the "largest evacuation in history".
If Irma hits the US it would be the first time two hurricanes of such strength have made landfall in the country for 102 years.
Authorities are still grappling with recovery efforts after Hurricane Harvey decimated parts of Texas and Louisiana two weeks ago.
Insurers are likely to pick up a larger proportion of the costs of Irma compared with Harvey because the damage is more likely to be as a result of strong winds than heavy rain.