Hundreds of thousands of people in Florida and South Carolina have been told to evacuate as Hurricane Matthew churns towards the US east coast.
“This is a serious storm,” warned President Barack Obama, adding that it would have a “significant” impact on Florida by Thursday morning.
Traffic jams are reported in the city of Charleston, in South Carolina, as residents flee the area.
The category three system is currently battering the Bahamas.
Matthew carved a trail of destruction across Haiti where thousands have been displaced and at least two people died.
The giant storm also hit Cuba. Early reports suggested its impact was not as hard as in Haiti but the town of Baracoa in the province of Guantanamo was badly hit, destroying many homes and scattering debris in the streets.
In Florida, mandatory and voluntary evacuations were taking place on Wednesday in vulnerable areas.
“We will likely start to see impacts on Florida within the next 24 hours and through the weekend,” Florida Governor Rick Scott tweeted.
“That means people have less then 24 hours left to prepare, evacuate and shelter.”
In a morning news conference he added: “If you’re able to go early, leave now.”
The Florida Division of Emergency Management said Matthew was forecast to move parallel to the coast on Thursday and Friday.
“Impacts will be dependent on the track Matthew takes; a more westward path would bring hurricane conditions to many east coast areas,” it warned.
The National Hurricane Center in Miami issued a storm surge warning for the coast from West Palm Beach north to Palm Bay.
In the neighbouring state of Georgia, Governor Nathan Deal issued a state of emergency order for 13 counties.
In South Carolina, authorities in Charleston have specified hurricane evacuation routes for residents to avoid congestion on the main Interstate 26 motorway. All lanes are being made outward to speed the flow of traffic.
To the west of Charleston, residents of Beaufort County have also been told to evacuate.
Governor Nikki Haley said that evacuations in two other areas, Georgetown and Horry counties, would begin on Thursday.
Schools along South Carolina’s Atlantic coast and in some central districts are to close.
In the Bahamas, Prime Minister Perry Christie urged residents in coastal areas to move to higher ground, the Nassau Guardian reported.
“If you live on the southern coast of any islands you will be exposed to risk,” he said.
He warned that the main island of New Providence was due to take “a direct hit”.
Cruise ships have been rerouted from their scheduled Caribbean and Bahamas ports to Key West in Florida.
In Haiti, at least 10,000 people were in shelters and there were reports of overcrowded hospitals suffering shortages of fresh water, Mourad Wahba, the UN special representative for Haiti, said.
The storm knocked down communications and blocked roads, hampering emergency efforts.
The collapse of a bridge cut off the only link between the capital, Port-au-Prince, and the southern part of the country, and officials said it would be difficult to reach the region.
In the port town of Les Cayes, the situation was “catastrophic”, with streets flooded and many houses without roofs, deputy mayor Marie Claudette Regis Delerme said. She herself had to flee a meeting when a gust ripped off the building’s roof.
Haiti is one of the world’s poorest countries, with many residents living in flimsy housing in flood-prone areas.
The UN said the country was facing the “largest humanitarian event” since a huge earthquake in 2010.
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