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.
“If there is an evacuation order in your community, you need to take it seriously,” US President Barack Obama said.
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 flights 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.
We only began to see the real destructive force of Hurricane Matthew once we moved towards the south-west of the country.
Trees fallen, banana crops uprooted and flattened, houses under water and men and women trying to get the debris out of the way.
It was noticeable how the people we passed were coping alone. There were no army or police around to help. Even the aid agencies are struggling to move around this damaged corner of the country.
“It’s not looking good,” said Jean Claude Fignole, programme director in Haiti for the aid agency Oxfam,
“We’re getting reports out of the southwest of Haiti… reports of schools that have been completely destroyed, many homes that have been destroyed.”
Matthew is the region’s most powerful hurricane since Felix in 2007.