Winds of up to 160mph smashed into Cuba's northern coast on Saturday hours after it was classified as a Category 5 storm.
Irma has now been downgraded to a Category 3, but is forecast to restrengthen once it moves away from Cuba,according to the US National Hurricane Centre.
Concerns are also mounting over Hurricane Jose, which is growing in strength and heading towards islands already devastated by Irma.
Florida is bracing for Irma to hit on Sunday morning. It is expected to bring massive wind and flooding damage to the fourth most populous US state.
Governor Rick Scott warned that Irma, the most powerful Atlantic Ocean hurricane in recorded history, is wider than the "entire state".
More than six million people - a quarter of the Florida's population - have been ordered to evacuate.
The Carolinas, Alabama and Georgia have also declared emergencies.
Foreign Secretary Boris Johnson said the UK was "taking swift action to respond" to the disaster after speaking to the chief minister of Anguilla, a British overseas territory that was among the first islands to be hit.
Britons in the region have been urged to follow evacuation orders, while states of emergency have been declared in Puerto Rico, Cuba and Florida - amid fears Miami could be struck directly by the hurricane.
A British naval ship has been deployed to help deal with the aftermath with 40 Royal Marines on board, as well as army engineers and equipment, as authorities struggle to bring aid to smaller islands.
The Antigua Meteorological Service has reiterated warnings for the Leeward Islands and British Virgin Islands, as a result of Hurricane Jose tracking North West.
The storm is blowing maximum sustained winds of 150mph.
The islands, along with a number of others, have already been badly damaged by Irma, which continues on its path to Florida.
There are claims that residents of the French side of St Martin are too scared to leave their homes to seek shelter from Hurricane Jose because of marauding gangs of looters
Jose is now Category 4 storm and threatens the island, which has already been battered by Hurricane Irma.
On Facebook groups discussing aftermath of the storm, residents reported hearing gunshots in the streets and have called for martial law.
St Martin is administered between the French and Dutch governments.
Yesterday, French government minister Annick Girardin told the BBC: "We need to restore public order to Saint-Martin.
"I was out this morning and this afternoon and there was looting right there in front of my eyes. There is a strange mood at the moment in Saint-Martin, so we need to think about public order."
Forecasters are predicting a storm surge of up to 12 feet on the Florida Keys. The National Weather Service has produced this graphic, showing how much water that could bring to the area.
Some incredible quotes from St Thomas on the US Virgin Islands from the Associated Press:
Laura and Taylor Strickling huddled with their year-old daughter in a basement apartment along with another family, as the storm raged for 12 hours.
"The noise was just deafening. It was so loud we thought the roof was gone," she said, adding that she and the three other adults "were terrified but keeping it together for the babies".
Ms Strickling, who used to visit her husband in Afghanistan when he worked there as a lawyer, added: "I've had to sit through a Taliban gunfight, and this was scarier."
When they emerged, they found their apartment was unscathed but the trees had no leaves.
"We're obviously worried by the thought of having to do it all again with Hurricane Jose. It's a little, a little, well, it's not good," she said, her voice trailing off.
Cuba has 'taken a beating' from Hurricane Irma, according to Hurricane Tracker App, and the storm has weakened. But it is expected to regain its strength as it moves over the warm waters of the Florida Straits.
There's been little information coming out of the communist island - where the media and internet are subject to heavy restrictions - but there have been reports of significant flooding from storm surges.
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