People have begun fleeing the coasts of Florida and South Carolina, stripping some stores of food, water and petrol as Hurricane Matthew continues its path towards the US.
Forced evacuations are under way in Florida’s Brevard and Martin counties, with governor Rick Scott saying the state could be facing “its biggest evacuation ever”.
A mass evacuation of a quarter of a million residents has also begun in South Carolina.
Both states, as well as Georgia and North Carolina, have issued a state of emergency.
The White House said emergency supplies had been moved to staging posts in the southeast of the country, while many shops have also seen a run on supplies such as propane, batteries and tarpaulin.
President Barack Obama warned: “I want to emphasise to the public this is a serious storm,”
“Even if you don’t get the full force of the hurricane we are still going to be seeing tropical force winds, the potential for storm surge and all of that could have a devastating effect.”
People living in coastal areas were warned by Florida’s governor “to leave now” and to stock up on several days’ worth of food, water and medicine.
He said people should be prepared for the possibility of a “catastrophic hurricane”.
The National Guard, other law enforcement officers and a fleet of 315 buses have been mobilised to support the evacuation of coastal counties in South Carolina.
Bumper-to-bumper traffic jammed Interstate 26 as people tried to get a head start on the 3pm start time for the evacuation in Charleston County and Beaufort.
Traffic flows have been reversed to allow more people to escape the coast.
The storm has already left a trail of destruction across the Caribbean, killing at least 26 people and slamming into Haiti with howling 145mph winds.
The peninsula battered most in Haiti has been cut off from the rest of the country, with floodwater running at shoulder height in places, and reserves of drinking water running low.
“What we know is that many, many houses have been damaged. Some lost rooftops and they’ll have to be replaced while others were totally destroyed,” interior minister Francois Anick Joseph said.
In the neighbouring Dominican Republic, four lost their lives. One person died in Colombia, and another in St Vincent and the Grenadines.
Dozens of homes were also destroyed in the eastern town of Baracoa in Cuba – and Matthew is now moving north and bearing down on the Bahamas
It is expected to hit the US by Thursday evening when a 5ft storm surge on the Atlantic coast could cause significant flooding. The hurricane could also trigger tornadoes.
Matthew’s vicious winds have weakened slightly to a 120mph Category 3 storm, but forecasters say that was expected and it will strengthen again in the coming days.
South Carolina governor Nikki Haley said people refusing to leave could be putting the lives of others at risk if they need help when the storm hits.
Some petrol stations have been running out of fuel in the area, and the state’s attorney general warned against stores bumping up prices to take advantage of people filling their vehicles to the brim.
In nearby Florida, where multiple hurricane warnings are in force, officials warned people against complacency – especially in the south of the state – after a recent 11-year break without a major storm, the longest on record.
People in the central Brevard County will be able to hunker down in shelters from 4pm local time on Wednesday, while patients in Cape Canaveral hospital have been moved inland.
The last storm to hit Florida from the Atlantic was Katrina in 2005, while Wilma – which hit with 120mph winds the same year – caused an estimated $21bn (£16bn) of damage and left thousands in the state without power.