Strong winds buffeted the southern coast of the Americas' most destitute country which is particularly vulnerable to natural disasters, in part because so many homes and buildings are flimsy.
Flooding now is being reported in some areas of Jamaica as the Category 4 storm creeps closer from the south, news reports said.
Cuba ordered the evacuation of more than 250,000 people from the east of the island.
"No one likes to leave their homes, but the sea is going to rise and that is very dangerous," said Pedro Gonzalez, a retired chef who had to leave a fishing islet where he lives off the city of Santiago.
With him went his sister Ana and their 100-year-old mother Marina, who uses a wheelchair.
"I would not stay on that cay for all the money in the world," said Ana, who says she recalls the horror of living through Hurricane Sandy in 2012. It killed 11 in Cuba.
Matthew is expected to hit southwestern Haiti Monday night, packing powerful winds and torrential rain, the US National Hurricane Center said in Miami.
Cuba and the Bahamas are also in the likely path of destruction.
Monstrous storm swells of up to 11 feet (3.3 meters) were forecast off the coasts Cuba and Haiti, the NHC said.
"This is shaping up to be a devastating blow," said Domenica Davis, a meteorologist for The Weather Channel.
The victims in Haiti were a fisherman who drowned Friday and another who went missing Sunday, both off the southern coast, civil protection officials said Monday.
At 1500, GMT the center of Matthew was located 275 miles (440 kilometers) southwest of Port-au-Prince, with lashing top wind speeds of 140 miles per hour, the US hurricane center said.
The storm was creeping forward at six miles (nine kilometers) per hour.
It is expected to continue north, tearing across southern and eastern Cuba between Monday and Tuesday as it marches toward the Bahamas.
Forecasts predict the hurricane will dump 15 to 25 inches (40 to 60 centimeters) of rain over southern Haiti with possible isolated maximum amounts of 40 inches.
"This rainfall will likely produce life-threatening flash floods and mudslides," the NHC warned.
Haiti, Jamaica, Cuba prepare
Thousands are still living in tents in Haiti after the country's massive 2010 earthquake. Erosion is also especially dangerous in Haiti because of high mountains and lack of trees and bushes in areas where they have been cut for cooking.
More than 500 people were evacuated Sunday as a precaution from Jeremie, in the southwest of Haiti. Some were reluctant to leave, said civil defense chief Marie Alta Jean-Baptiste.
"My countrymen, do not be stubborn, do not say 'God is good' and will take care of you," interim president Jocelerme Privert said in an address to the nation.
"The dangerous areas will be evacuated. We have no interest in risking your lives," he said.
The poorest country in the Americas is home to almost 11 million people, many living in fragile housing.
In Jamaica, officials said the army and military reserves were called up to help deal with hurricane damage.
Buses were also being sent to flood-prone areas to move residents to shelters.
US embassies in Jamaica and Haiti closed Monday and Tuesday due to the storm.
In Cuba, President Raul Castro traveled to the southeastern city of Santiago to oversee emergency operations.
Matthew had the potential to become a storm for the ages, he warned residents.
"This is a hurricane it's necessary to prepare for as if it were twice as powerful as Sandy," the Cuban leader said, referring to the megastorm that hit with massive destructive force in 2012.
At the US naval base in Guantanamo Bay, Cuba, located in the area where Matthew is expected to cross, a mandatory evacuation was ordered for all non-essential personnel and family members.
America's Accuweather website warned meanwhile that Matthew could hit the US East Coast around midweek.
