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.
Irma's eye passed directly over the island of Barbuda, meaning its inhabitants were twice subjected to the fastest winds.
Hurricane Irma – devastation caused in pictures
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Heavy rain and howling winds raked the neighbouring island of Antigua, sending debris flying as people huddled in their homes or government shelters.
Officials had warned people to seek protection from Irma's "onslaught" in a statement that closed with: "May God protect us all."
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 southern tip of Florida is already experiencing 45-55mph gusty showers from the outer edges of Hurricane Irma, according to the National Weather Service.Â
Weather World's Steve Seman warned that the storm – recently downgraded to Category 3 – still has more than triple the energy of Hurricane Andrew, which wreaked havoc on Florida in 1992.
The storm is expected to gain strength as it moves toward the US.
Weather conditions in Cuba continue to deteriorate, even as Hurricane Irma travels toward the US mainland.
The storm submerged the town of Caibarien, Cuba under several feet of water, according to CNN's Patrick Oppman. The capital city of Havana is preparing for "impending coastal floods" according to local news outlets.
Severe winds have damaged homes, hospitals, and government buildings. More than 20 people have been killed by the storm across the Caribbean.
The Dutch government estimates 70 percent of houses on St. Maarten were badly damaged or destroyed by Hurricane Irma. That leaves many of the 40,000 residents reliant on public shelters as they brace for Hurricane Jose.Â
Interior Minister Ronald Plasterk said Saturday that Jose is forecast to track northwest of St. Maarten and will likely dump a lot of rain on buildings, many of which had roofs torn off by Irma.Â
Dutch Prime Minister Mark Rutte said the situation remains "grim" on the island where looting has broken out.Â
Rutte says there are some 230 Dutch troops and police patrolling St. Maarten and a further 200 will arrive in coming days. Rutte issued a warning to looters that the troops and police will clamp down hard to end the lawlessness.Â
Cuban officials say Hurricane Irma has damaged crops in the rural eastern part of the country having smashed through parts of the island.Â
Civil Defense official Gergorio Torres tells reporters that authorities are still trying to tally the extent of the damage in Las Tunas province and nearby areas. He said damage seems to have been concentrated in infrastructure for crops including bananas.Â
As Irma tracks westward new cities on in western Florida are at risk of a direct hit.  Tampa has not been struck by a major hurricane since 1921, when its population was about 10,000, National Hurricane Center spokesman Dennis Feltgen said. Now the area has around 3 million people.
One group who are certainly not staying away from Florida are electricians.
Nick Chilelli had been driving his utility truck with a broken air conditioner for about 18 hours, all the way from Cincinnati, when his convoy got trapped behind a car wreck just outside Orlando, Florida, at about 2am Saturday morning.
They were among thousands of line workers racing through the night to stage their trucks and tools for Hurricane Irma, a monstrous storm expected to knock out power to half of the nation's third most-populous state.
They had been told to expect to work at least a month of straight 16-hour days, with no breaks, trying to restore power to millions of homes.
"I'll probably make 30 grand this month," Mr Chilelli, 48, told Reuters. "Everybody out here is killing it. Of course, you're dealing with something that could kill you any minute."
Storm surges have been the danger that Florida's officials have been warning about. Waves will be at least 5ft higher in Miami come Sunday, while Fort Myer and up to Tampa on the western coast of Florida could see waves 10ft to 15ft higher.
While the storm will pass over, officials are warning that residents should stay off the streets with surges providing another danger after the winds of Irma.
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