ALSO READHurricane Irma batters Caribbean, Florida braces for powerful storm Hurricane Irma wreaks havoc in Barbuda, to hit Florida in 3 days Hurricane Irma hits an already crisis-ridden Puerto Rico Hurricane Irma kills 10 in Caribbean, heads for Florida Hurricane Irma most enduring super-storm on record: Weather service
Hurricane Irma pounded more of the Caribbean, shredding homes and weather records and leaving at least 12 people dead as it honed in on the United States, where up to a million people were told to flee.
The evacuation of coastal areas of Florida and neighbouring Georgia was the biggest seen in the US in a dozen years, as Brock Long, head of the US Federal Emergency Management Agency, warned: "It will be truly devastating.
"The entire southeastern United States better wake up and pay attention."
Barreling across the Caribbean, the rare Category Five Irma wielded monster winds and torrential rain, wreaking destruction on tiny islands like St Martin, where 60 percent of homes were wrecked, before slamming into the US Virgin Islands and Puerto Rico.
In its westward rampage, Irma packed winds of up to 185 miles per hour (295 kilometers per hour), an intensity that it sustained for 33 hours -- the longest of any storm since satellite monitoring began in the 1970s.
The latest bulletin from the Miami-based National Hurricane Center included the first hurricane and storm surge warnings for south Florida and the Florida Keys, even as winds slowed slightly to 165 mph.
The storm left devastation in its wake. The International Red Cross said 1.2 million people had already been hit by Irma, a number that could rise to 26 million.
On many islands, roofs were ripped off buildings as if by a giant's hand, shipping containers were tossed aside like matchsticks and debris flung far and wide, and airports, sea ports and mobile phone networks were knocked out.
At least two people were killed in Puerto Rico, a senior rescue official said. More than half of the territory's population of three million was without power, with rivers breaking their banks in the center and north of the island. Governor Ricardo Rossello activated the National Guard and opened storm shelters sufficient for up to 62,000 people.
Authorities on the US Virgin Islands also reported four deaths.
"We lost a significant and a good number of assets... In terms of fire stations, police stations," Governor Kenneth Mapp said in a Facebook post, adding that the region's main health facility, the Schneider Regional Medical Center, lost its roof.
Lisa Poser, a spokeswoman for Mapp, told AFP the territory's critically wounded were transported to Puerto Rico and warned there may be more casualties as authorities continue their search and rescue.