Hurricane Matthew, the fiercest Caribbean storm in almost a decade, is powering toward the Bahamas and Florida's eastern coast after battering Haiti and Cuba with torrential rains and killing at least 17 people.
The hurricane whipped the two island nations with 230km/ph winds, which the United Nations said created the worst humanitarian crisis to hit Haiti since a devastating 2010 earthquake.
In the US, millions of people were urged to evacuate the southeastern coast and Florida Governor Rick Scott warned residents to prepare for a possible direct hit that could be catastrophic.
Hundreds of thousands of people had been evacuated from the path of the hurricane, which caused severe flooding and killed four people in the Dominican Republic as well as at least 13 in Haiti, the two countries that share the island of Hispaniola.
The storm carved a path of devastation through western Haiti, destroying houses, dumping boats and debris on coastal roads and heavily flooding residential areas.
Matthew was a Category Four hurricane on Tuesday but was downgraded to Category Three early on Wednesday, the Miami-based National Hurricane Center said.
Maximum sustained winds eased to about 193km/ph by Wednesday morning but the NHC said the hurricane was likely to strengthen again slightly in the coming days.
The eye of the storm was about 169km south of Long Island in the Bahamas and it was expected to be very near the east coast of Florida by Thursday evening.
"Everyone in our state must prepare now for a direct hit," Governor Scott said.
"If Matthew directly impacts Florida, the destruction could be catastrophic and you need to be prepared."
It was difficult to assess the severity of Matthew's impact in Haiti because it knocked out communications in many of the worst-affected areas, including the main bridge that links much of the country to the southwest peninsula.
Impoverished Haiti was a particular concern because tens of thousands of people are still living in tents and makeshift dwellings due to the 2010 earthquake that killed more than 200,000 people.