The fiercest Caribbean storm in almost a decade has battered Haiti's southwest coast with 230km/h winds and surging seas that flooded coastal towns, tore at trees and rooftops before moving out to sea.
Hurricane Matthew dumped torrential rain on Tuesday 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.
There was no immediate word on other potential casualties. But Pentagon spokesman Peter Cook told reporters in Washington the US Navy was looking at possibly sending an aircraft carrier, the USS 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 impoverished country was apparently extensive.
"It is too early to do a real assessment, but it has been very serious," Anick Joseph told Reuters, saying roads to the south were blocked, phones were down and the weather meant overflights were impossible.
The eye of the violent Category 4 Hurricane Matthew passed over Haiti's western tip early on Tuesday, the US National Hurricane Center said, bringing devastating winds, heavy rains and a storm surge with massive waves.
It was forecast to remain extremely powerful as it made its way to Cuba and the Bahamas and a hurricane watch was issued for parts of southeast Florida, which the forecasters said Matthew could reach late on Thursday.
US President Barack Obama cancelled a trip to Florida scheduled for Wednesday because of the potential impact of Matthew, the White House said.
Haitian port town Les Cayes, named for the sandy islands off its shore and twice destroyed by hurricanes in the 18th century, was hard hit.
"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 and taken away their rooftops," said Deputy Mayor Marie Claudette Regis Delerme, speaking from the town of about 70,000 people.
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 ill 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.
Overnight, Haitians living in vulnerable coastal shacks on the western Tiburon Peninsula frantically sought shelter as Matthew closed in. Several districts in southern Haiti were flooded, with crops inundated with ocean and rain water.
Hurricane conditions remained in parts of Haiti and as much 1 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 shantytown 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 storm is expected to hit the tourist town of Baracoa in the province of Guantanamo in the far southeast of Cuba later on Tuesday, 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 of service personnel as the storm approached.
Guantanamo's mountainous terrain is the country's second coffee producer after nearby Santiago, and the storm poses a major threat to the current harvest.
A hurricane watch was in effect from Deerfield Beach, Florida to the Volusia-Brevard county line, a coastal area near Cape Canaveral, which the storm could reach on Thursday, the hurricane centre said.
"Direct hurricane impacts are possible in Florida," the centre said. It added that tropical storm or hurricane conditions could also affect parts of Georgia, South Carolina, and North Carolina later this week, even if the center of Matthew remained offshore.
Governor Rick Scott declared a state of emergency for Florida on Monday, designating resources for evacuations and shelters and putting the National Guard on standby.
