Residents in Haiti and Jamaica have been ordered to evacuate as Cuba suspended flights in preparation for the worst storm to hit the Caribbean in almost a decade.
Hurricane Matthew has hit western Haiti with 145 mile-per-hour (230 kph) winds, killing at least one person and damaging homes.
Residents of coastal areas in Haiti and Jamaica had been urged to evacuate their homes before the eye of the Category 4 storm made landfall near Les Anglais at 1100 GMT. Matthew is the strongest storm to hit the region since 2007.
About one metre of rain is forecast to fall over denuded hills prone to flash floods and mudslides, threatening villages as well as shanty towns in the capital Port-au-Prince, where heavy rain fell overnight.
"We are very worried by the situation," Haitian Interior Minister Francois Anick Joseph said at an operations centre in Port-au-Prince, adding that 1,300 shelters had been set up, with the capacity to hold 340,000 people.
Some 2,000 people refused to leave their seaside homes in the coastal town of La Savanne and the government was ready to use force if needed, Joseph said.
Poor Haitians are often reluctant to leave home in the face of storms, fearing their few belongings will be stolen.
The cyclone comes at a bad time for Haiti, where tens of thousands of people still live in tents after a 2010 earthquake that killed upwards of 200,000 people.
Cholera introduced by UN peacekeepers is expected to rise in the October rainy season, and the country was due to hold a long-delayed presidential election on Oct. 9.
The office of Interim President Jocelerme Privert said there was no change to the election date.
Jamaica and Cuba brace for distaster
In Kingston, Jamaica, major roads and waterways flooded as the first bands from Matthew lashed the island.
Cars stalled as rain-drenched drivers tried to push vehicles through streets that flooded within minutes after the downpour started.
Kingston residents stocked up on canned food, water and batteries, while banks and offices boarded their windows.
Fishermen were told not to go to sea.
Flights in Cuba were suspended on Sunday with residents queuing up in long lines for supplies.
President Raul Castro traveled to the southeastern city of Santiago to oversee emergency operations just hours before the hurricane was due to hit.
"This is a hurricane that is necessary to prepare for as if it were twice as powerful as Sandy," the Cuban leader said, referring to the mega storm that hit with massive destructive force in 2012.