Hurricane Matthew made landfall in Haiti on Tuesday as an "extremely dangerous" Category 4 storm, unleashing life-threatening winds and torrential downpours on the poorest nation in the Americas.
The US National Hurricane Centre said the storm's eye passed over Les Anglais, near the tip of Haiti's south-western peninsula, at 7 am (1200 GMT) with maximum sustained winds of 230 kilometres per hour.
After crossing over Haiti, the hurricane's present course has it approaching Cuba by late Tuesday. By Wednesday forecasters said Matthew could begin striking parts of the Bahamas.
Caribbean nations have been on alert since Matthew strengthened into a hurricane in late September and then briefly intensified into a Category 5 storm at the weekend, becoming the strongest in the Atlantic since 2007.
Evacuations have taken place across Jamaica, Haiti, Cuba and elsewhere, with residents of low-lying areas told to leave their homes and to expect storm surges, flash floods and mudslides.
Parts of Haiti could experience 101 centimetres of rainfall, the US hurricane centre said.
The World Meteorological Organization in Geneva warned Tuesday that Haiti is the nation most vulnerable to Hurricane Matthew. It noted that the last Category 4 storm was Hurricane Flora in 1963, which was one of the deadliest hurricanes to hit the region.
About 80 per cent of Haiti's population lives below the poverty line, according to US statistics, many residing in flimsy shelters susceptible to toppling.
The UN World Food Programme (WFP) said it was preparing to come to the aid of affected countries.
In Haiti, WFP has enough emergency food stockpiled in positions across the country to feed up to 300,000 people for one month.
"Our priority is to support the governments' interventions to save lives and meet the food needs of the most vulnerable and food insecure people affected," said Miguel Barreto, WFP Regional Director for Latin America and the Caribbean, in a statement.
The US has sent disaster response teams to Jamaica and Haiti the White House said.
At least three deaths - one each in Haiti, Colombia, and Saint Vincent and the Grenadines - have been connected to the hurricane, according to local officials and media reports.
Cuban authorities have evacuated about 1 million people from coastal regions, while the US Navy has ordered the evacuation of about 700 non-essential staff from its base at Guantanamo Bay.
The governor of the US state of Florida has declared a state emergency. Matthew is forecast to arrive there later in the week.