The most powerful Caribbean hurricane in nearly a decade has hit Haiti, bringing 230km/h (145mph) winds, heavy rain and dangerous storm surges.
Hurricane Matthew, a Category Four storm, has led to a number of deaths in Haiti and the Dominican Republic.
Southern Haiti has effectively been cut off after the bridge linking it to the capital, Port-au-Prince, collapsed.
Matthew has now moved off the north-eastern coast of Cuba towards Florida, where warnings are in place.
Early reports suggested that Cuba had not been hit as hard as Haiti.
Two people are known to have died in Haiti and four in the neighbouring Dominican Republic.
The death toll is expected to rise as authorities assess the extent of the damage.
The situation was described as "catastrophic" in the port town of Les Cayes.
Deputy Mayor Marie Claudette Regis Delerme said the city of 70,000 people was flooded and many houses had lost roofs. She herself had to flee a meeting when a gust ripped off the building's roof.
Images showed people walking in shoulder-high water, with relief workers saying that other coastal communities were also under water, including Les Anglais.
The mayor of Les Cayes, Jean Gabriel Fortune, posted tweets that included video showing damage in the Vernet area.
Fonie Pierre, director of Catholic Relief Services for Les Cayes, told AP: "Many people are now asking for help, but it's too late because there is no way to go evacuate them."
Officials said they were trying to restore access to Haiti's southern peninsula after the collapse of the La Digue bridge but admitted it would be difficult to find an alternative route.
Haiti is one of the world's poorest countries, with many residents living in flimsy housing in flood-prone areas.
The UN said the country was facing the "largest humanitarian event" since a huge earthquake in 2010.
At least 10,000 people were in shelters and there were reports of overcrowded hospitals suffering shortages of fresh water, Mourad Wahba, the UN special representative for Haiti, said.
More than four million children could be exposed to hurricane damage, Unicef said, warning of the spread of waterborne disease.
Haiti's Interim President Jocelerme Privert said earlier that some people at sea or who had not "respected alerts" had died, but he gave no more details.
Little protection: Nick Bryant, BBC News, Fonds-Parisien, Haiti
Haiti has been taking a brutal pounding. The conditions here are atrocious - to step outside is to be drenched within seconds.
Most vulnerable have been those who inhabit the shacks along the western coastline. There, storm surges are the main danger.
People here are confronting life-threatening flooding and the likelihood of mudslides in a landscape denuded of trees.
This Category Four storm comes as this impoverished country is still recovering from the 2010 earthquake which killed more than 200,000 people, and a cholera epidemic that followed.
Many live in shanty towns that offer little protection from the high winds and rains. And many refused to evacuate, fearing that what few possessions they have left would be stolen.
A hurricane alert was in place for five provinces of Cuba, with residents moved from low-lying areas.
Six-metre waves were reported on the seafront in the eastern town of Baracoa but state media said there were no immediate reports of serious damage.
Some 700 spouses and children of US service members were flown out of the Guantanamo Bay base.
It is also predicted to hit the US east coast later in the week. South-eastern Florida was under hurricane and tropical storm warnings.
South Carolina would start evacuating more than a million people from its coastline on Wednesday, Governor Nikki Haley said.
Hurricane Matthew is the region's most powerful since Felix in 2007.
Category one: sustained winds of 74-95mph (119-153 km/h); some damage and power cuts
Category two: winds of 96-110mph (154-177 km/h); extensive damage
Category three: winds of 111-129mph (178-208 km/h); well-built homes suffer major damage
Category four: winds of 130-156mph (209-251 km/h); severe damage to well-built homes, most trees snapped or uprooted
Category five: winds of 157 mph (252 km/h) or higher; high percentage of homes destroyed, area uninhabitable for weeks or months
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