By Makini Brice and Sarah Marsh
LES CAYES, Haiti/GUANTANAMO, Cuba (Reuters) - Hurricane Matthew, the fiercest Caribbean storm in almost a decade, powered toward the Bahamas and Florida early on Wednesday after battering Haiti and Cuba with torrential rains and killing at least 10 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.
Hundreds of thousands of people were evacuated from the storm, which caused severe flooding and killed four people in the Dominican Republic as well as at least six in Haiti, the two countries that share the island of Hispaniola.
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.
The eye of the storm was about 115 miles (185 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.
Maximum sustained winds eased to around 115 mph (185 kph) by Wednesday morning but the NHC said it was likely to strengthen again slightly in the coming days.
It was difficult to assess the severity of the impact on Haiti because Matthew knocked out communications in many of the worst-affected areas, including the main bridge that links much of the country to the southwest peninsula.
There was particular concern about Haiti as tens of thousands of people are still living in tents and makeshift dwellings due to the 2010 earthquake, which killed more than 200,000 people.
Authorities said on Wednesday two people in Haiti were crushed by trees and two were swept away by swollen rivers.
Mourad Wahba, the U.N. 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 U.S. 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 in Cuba of deaths, casualties or major damage.
Cuba's government has traditionally made extensive efforts to cope with hurricanes and authorities spent days organizing volunteers to get residents to safety and secure property.
Matthew thrashed the tourist town of Baracoa in the province of Guantanamo, passing close to the disputed U.S. Naval base and military prison. It 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 had been 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 Nikki 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.
(Additional reporting by Brendan O'Brien in Milwaukee and Marc Frank in Cuba; Writing by Dave Graham; Editing by Simon Gardner and Bill Trott)