Video report by ITV News Washington correspondent Robert Moore
Hurricane Irma has made landfall in Florida Keys, bringing winds of up to 130mph.
Millions of people have been told to leave as the storm - now upgraded to Category 4 - blasted through Cuba towards to the US, shifting its path to target the west of the state.
Police in Florida have reported a number of road traffic deaths in the last few hours. It is not yet clear whether they were caused by the adverse weather.
Florida utility officials say more than 1 million customers have lost power as the hurricane hit the state.
Irma has already devastated parts of the Caribbean, leaving at least 22 people dead, and causing destruction in the UK territories of Turks and Caicos and the British Virgin Islands.
British police officers will be deployed along with almost 500 troops as part of the country's efforts to step up support to the Caribbean islands.
Irma battered Cuba on Saturday with wind speeds reaching up to 160 mph.
Donald Trump called on "everyone in the storm path to heed all instructions, get out of its way".
Officials have warned Irma will bring "life-threatening wind, storm surge and rainfall" when it arrives at the Florida Keys and Tampa on Sunday.
"You need to leave now, do not wait," said Florida Governor Rick Scott. "Evacuate. Not tonight, not in an hour, you need to go right now."
Mr Scott said there could be a "six to 12 foot" storm surge. "This will cover your house," he said.
A total of 6.5 million people - a third of the state's population - have been urged to leave voluntarily, while mandatory evacuations have been declared in some areas.
Thousands more face being caught up in the hurricane as Irma shifted westward, meaning that its centre is now expected to move over the west side of the state's peninsula, rather than the east as originally forecast.
Video report by ITV News Correspondent Martin Geissler
Governor Scott warned those in the low-lying Keys to get out within hours or be prepared to weather the storm on their own as the authorities will not risk sending emergency services into the hurricane's centre.
"If you are planning to leave and do not leave tonight, you will have to ride out this extremely dangerous storm at your own risk," he said.
The biggest evacuation in Miami's history has left its streets eerily quiet as the city braces itself to face the eye of the storm.
Some of the city's shelters are already full.
What is happening in Cuba?
Irma is the first Category Five hurricane to hit Cuba for more than 80 years.
The storm battered the island on Saturday with deafening winds and relentless rain, pushing seawater inland that flooded homes and knocked out power across a wide area.
High winds upended trees, toppled utility poles and scattered debris across streets.
Roads were blocked, and witnesses said a museum near the eye of the storm was in ruins after being buffeted by brutal squalls.
Early on Saturday soldiers and government workers went through coastal towns enforcing an evacuation, taking people to shelters at government buildings, schools and even caves.
What is the situation in the rest of the Caribbean?
Many Caribbean residents and tourists were left reeling after Irma ravaged some of the world’s most exclusive tropical playgrounds
Some islands remain without power, water or telephone services and are now preparing for a second pounding from Hurricane Jose, a Category Four storm, which is set to hit the islands over the weekend.
The 22 killed by Irma include 11 on St Martin and St Barts, four in the US Virgin Islands, four in the British Virgin Islands and one each on Anguilla and Barbuda.
Officials on the French overseas islands of St. Martin and St. Barts said: “The protection and shelter of people already harshly tested by Irma is the priority."
More than 1,100 police, military officials and others have been deployed to St. Martin and St. Barts to provide help and crews were evacuating the sick and injured to nearby Guadeloupe.
Hurricane Jose has left its original path through the Caribbean, instead veering north-west.
Hurricane warnings were in effect for Dutch Sint Maarten, St. Martin and St. Barthelemy, and tropical storm warnings were in effect for Barbuda and Anguilla, as well as Saba and St. Eustatius.
Looting and gunshots have been reported on St Martin, while a curfew has been imposed in the US Virgin Islands.
What is the British government response?
British police officers will provide support to British Army soldiers who have been deployed from RFA Mounts Bay on the British Virgin Islands.
The UK has just under 500 troops currently in the region, made up of marines, engineers, and a number of medics and specialists, including Army and RAF personnel.
The National Police Chiefs' Council said two members of the UK police cadre, who support the military in times of international crisis, flew out on Friday, while a further 53 British officers from 14 police forces, including the Metropolitan Police, are due to leave from RAF Brize Norton in co-operation with the MoD.
The officers will support the local police force to maintain law and order, as well as helping to find missing people, including British nationals.
Following criticism of its response to the disaster, the Government announced a £32 million aid package and pledged to double any public donations to the British Red Cross appeal for victims of Hurricane Irma, up to £3 million.
Prime Minister Theresa May said the British government was working with US authorities to ensure British expats and tourists were protected in the Sunshine State.
Following a meeting of the government's emergency Cobra committee, Mrs May said: "I heard directly from our consul general in Miami about the support that is being given to British nationals living in Florida and also British tourists in Florida.
"We are, of course, working with the US authorities to ensure that every support is available and everything can be done before Hurricane Irma reaches Florida."
But the Foreign and Commonwealth Office said the "situation could deteriorate significantly" and its "ability to provide assistance may be extremely limited".