Rescue workers in Haiti are struggling to reach parts of the country cut off by the most powerful Caribbean hurricane in nearly a decade.
The destruction wrought by Hurricane Matthew has forced Haiti's presidential election, due to take place at the weekend, to be postponed, officials say.
Thousands have been displaced and at least five have died in the country.
The US state of Florida is bracing for the storm's arrival late on Thursday.
The storm is currently over the Bahamas, where all air and sea traffic has been halted and people were urged to move to higher ground.
Matthew battered a remote area of Haiti - one of the world's poorest countries, with many residents living in flimsy housing in flood-prone areas - with winds of up to 230km/h (145mph) on Tuesday.
Officials said they were not yet in a position to gauge the true extent of the damage - particularly in the Grand Anse area on the southern tip of the island, which was directly in the storm's path.
A key bridge has been destroyed, roads are impassable and phone communications are down, officials said.
Mourad Wahba, the UN special representative for Haiti, said at least 10,000 people were in shelters and hospitals were overflowing.
"What we know is that many, many houses have been damaged," Interior Minister Francois Anick Joseph said. "Some lost rooftops and they'll have to be replaced, while others were totally destroyed."
Among the damaged buildings were schools and churches due to be used as polling stations in Haiti's election. A new date for the much-delayed vote has not yet been announced.
Flight from Florida
At least 11 people are believed to have been killed by Hurricane Matthew so far, including four killed by collapsing walls and mudslides in the Dominican Republic.
In Cuba, dozens of homes in the eastern city of Baracoa were destroyed.
In the US, President Barack Obama warned that the coming days would be dangerous and urged Americans to heed official advice.
Florida Governor Rick Scott said his state could be facing its biggest evacuation ever.
South Carolina's Governor Nikki Haley said 250,000 people would be evacuated from coastal areas.
Matthew is the region's most powerful hurricane since Felix in 2007.