Matthew is the Caribbean's strongest storm in nearly a decade. Photograph: (AFP)
The storm destroyed houses, dumping boats and debris on coastal roads and heavily flooding residential areas in western Haiti
Hurricane Matthew, the fiercest Caribbean storm in almost a decade, powered toward the Bahamas and Florida's eastern coast early on Wednesday after battering Haiti and Cuba with torrential rains and killing at least 17 people.
The hurricane, which the United Nations said created the worst humanitarian crisis to hit struggling Haiti since a devastating 2010 earthquake, whipped Cuba and Haiti with 140 mile-per-hour (230 kph) winds on Tuesday, pummeling towns, farmland and resorts.
In the United States, 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 Matthew, 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 through Tuesday but was downgraded to Category Three early on Wednesday, the Miami-based National Hurricane Center (NHC) said.
Maximum sustained winds eased to about 120 mph (193 kph) by late 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 105 miles (169 km) south of Long Island in the Bahamas on Wednesday morning and it was expected to be very near the east coast of Florida by Thursday evening, the NHC said.
"Everyone in our state must prepare now for a direct hit," Scott said at a news conference in Tallahassee. "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 still are living in tents and makeshift dwellings due to the 2010 earthquake that killed more than 200,000 people.
Authorities said on Wednesday five people in Haiti were crushed by trees and six were swept away by swollen rivers.
"More than 1,000 homes have been damaged or destroyed by the flood waters and violent winds," said Ernst Ais, the mayor of the town of Cavaillon, near Les Cayes. He said a mother and three children died in floodwaters in his town.
Three men were killed in Leogane, near Port-au-Prince, when a coconut palm fell on their home, the mayor there said.
Mourad Wahba, the UN secretary-general's deputy special representative for Haiti, said much of the population had been displaced by Matthew and at least 10,000 were in shelter.
"Haiti is facing the largest humanitarian event witnessed since the earthquake six years ago," he said.
Heifer International, a nonprofit organization working with farming families in Haiti, said farmland and businesses caught in Matthew's path had been devastated.
The US government said it was ready to help the afflicted and about 300 U.S. Marines set off on the USS Mesa Verde to provide disaster relief in Haiti, the Marines said in a tweet.
There were no immediate reports of deaths, casualties or major damage in Cuba, where the government emphasized hurricane preparation. But Matthew did thrash the tourist town of Baracoa in the province of Guantanamo, gutting many houses and dumping hunks of cement, wooden beams, roof tiles and fallen electrical lines on the streets.
The storm passed close to the disputed U.S. Naval base and military prison, and was on track to mow over the central and northwestern Bahamas, the NHC forecasts showed.
Santiago de Cuba, the second-largest city, which lies in the southeast of Cuba, was not badly hit, state media said.
Officials in the Bahamas urged residents to evacuate to higher ground and the Ministry for Grand Bahama said on Facebook that government offices in New Providence and Grand Bahama were closed until further notice.
Hurricane and tropical storm warnings were extended along the east coast of Florida as the storm moved north.
South Carolina Governor Nikkei Haley on Tuesday declared a state of emergency and ordered the evacuation of more than 1 million people from Wednesday afternoon.
Tropical storm or hurricane conditions could affect parts of Georgia, South Carolina, and North Carolina later this week, even if the center of Matthew remained offshore, the NHC said. Scott told residents to brace for tornadoes, strong winds, rip currents and flooding and urged those along the state's eastern coast to make plans to evacuate.