Florida residents have been warned to prepare for a "direct hit", as Hurricane Matthew pounded the Bahamas after devastating parts of Haiti.
Damage could be "catastrophic", the state governor said. Evacuation orders have been issued along the coast.
Matthew, with winds of 125mph (205km/h), is expected to strengthen and hit Florida as a Category Four storm, US officials say.
It has swept across the Caribbean, with the worst of the damage in Haiti.
At least 22 people have been killed in Haiti and thousands displaced. The storm has forced the presidential election there to be postponed.
Four people were also killed in the neighbouring Dominican Republic.
As of 09:00 GMT, the storm was strengthening over north-western Bahamas. All air and sea traffic has been halted and people urged to move to higher ground because of storm surges.
Hurricane Matthew was expected to pass near Grand Bahama Island and "move very close" to the east coast of the Florida peninsula on Thursday night, the US National Hurricane Center (NHC) said.
"Everyone in our state must prepare now for a direct hit," Florida Governor Rick Scott told a news conference.
"If Matthew directly impacts Florida, the destruction could be catastrophic and you need to be prepared."
Some two million people have been advised to evacuate across coastal areas of Florida, Georgia and South Carolina. North Carolina could also be affected as the storm moves north.
US President Barack Obama said: "If there is an evacuation order in your community, you need to take it seriously."
Florida's history of hurricanes
- October 2005: Hurricane Wilma Deaths were reported in Haiti, Mexico, Cuba. Struck Florida as a Category Three, killing five people. South Florida suffered widespread damage
- August 2004: Hurricane Charley Struck Florida as a Category Four, killing nine and causing billions of dollars in property damage
- August 1992: Hurricane Andrew Category Five storm struck South Florida, killing 44 and damaging more than 125,000 houses
In many areas cars queued for fuel and residents stocked up on food and hardware supplies to board up their houses, local reports said.
Schools, universities and government offices in some areas are closed. Flights are disrupted, with American Airlines cancelling all services in and out of three South Florida airports.
In South Carolina, lanes on some highways have been reversed so vehicles can head west away from the storm.
In Haiti, meanwhile, rescue workers are struggling to reach areas cut off by the storm.
Officials said they were not yet in a position to gauge the true extent of the damage - particularly in the Grand Anse area, which was directly in the storm's path.
A key bridge had been destroyed, roads were impassable and phone communications were down, officials said.
Mourad Wahba, the UN special representative for Haiti, said at least 10,000 people were in shelters and hospitals were overflowing.