The fiercest Caribbean storm in almost a decade battered Haiti's southwest coast with 230km/h winds and surging seas that flooded coastal towns, tearing at trees and rooftops before moving on to Cuba.
Hurricane Matthew dumped torrential rain across the island of Hispaniola that Haiti shares with Dominican Republic, where four people were killed by collapsing walls and mudslides.
In Haiti, at least one person was killed.
By late afternoon, the cyclone was pounding the eastern tip of Cuba, the US National Hurricane Center said.
In Cuba's city of Guantanamo, about 160km from the storm's core, the winds picked up and streets emptied as people moved to shelters or inside their homes.
Matthew is predicted to maintain Category 4 strength through the Bahamas and weaken slightly as it tracks towards Florida later in the week.
Cameras outside the space station captured views of major Hurricane Matthew as it flew 250 miles over the massive storm today. pic.twitter.com/XoLpXvfkKd
— Intl. Space Station (@Space_Station) October 4, 2016
With communications out across most of Haiti and a key bridge impassable because of a swollen river, there was no immediate word on the full extent of potential casualties and damage from the storm in the poorest country in the Americas.
But Pentagon spokesman Peter Cook told reporters in Washington the US Navy was looking at possibly sending an aircraft carrier, the George Washington, and other ships to the region to aid relief efforts.
The United States has already offered Haiti the use of some helicopters, said Haitian Interior Minister Francois Anick Joseph, who added that damage to housing and crops in the country was apparently extensive.
"It is too early to do a real assessment, but it has been very serious," Anick Joseph said.
Named for the sandy islands off its shore and twice destroyed by hurricanes in the 18th century, the Haitian port town Les Cayes was hit hard by Matthew.
We need your consent to load this rte-player contentWe use rte-player to manage extra content that can set cookies on your device and collect data about your activity. Please review their details and accept them to load the content.Manage Preferences
"The situation in Les Cayes is catastrophic. The city is flooded. You have trees lying in different places and you can barely move around, the wind has damaged many houses," said Deputy Mayor Marie Claudette Regis Delerme, who fled a house in the town of about 70,000 when the wind ripped the roof off.
One man died as the storm crashed through his home in the nearby beach town of Port Salut, Haiti's civil protection service said.
He had been too sick to leave for a shelter, officials said.
A fisherman was killed in heavy seas over the weekend as the storm approached, and another was missing.
Hurricane conditions remained in parts of Haiti and as much as one metre of rain was forecast to fall over hills that are largely deforested and prone to flash floods and mudslides, threatening villages as well as shantytowns in the capital Port-au-Prince.
The hurricane comes at a time when tens of thousands of people are still living in flimsy tents and makeshift dwellings in Haiti after a 2010 earthquake that killed more than 200,000 people.
The center of Matthew was 45km south of the eastern tip of Cuba at 2100 GMT.
It was moving north at about 17km/h, the hurricane centre said.
Cuba traditionally puts extensive efforts into saving lives and property in the face of storms, and authorities have spent days organising teams of volunteers to move residents to safety and secure property.
The storm is expected to hit the tourist town of Baracoa in the province of Guantanamo, passing close to the disputed US Naval base and military prison and also to a small Cuban city of the same name.
The US Navy ordered the evacuation of 700 spouses and children along with 65 pets of service personnel as the storm approached.
South Carolina Governor Nikki Haley declared a state of emergency and ordered an evacuation of more than a million people in coastal areas starting tomorrow afternoon in anticipation of Matthew.
US President Barack Obama had earlier cancelled a trip to Florida scheduled for tomorrow because of the potential impact of the storm, the White House said.
A hurricane watch was in effect for Florida from an area just north of Miami Beach to the Volusia-Brevard county line, near Cape Canaveral, which the storm could reach on Thursday, the hurricane centre said.
