Hurricane Matthew bore down on Haiti, where towns and villages braced for "catastrophic" floods and mudslides.
Forecasters fear that the hurricane will bring 220km/h winds and up to 90cm of rain.
Matthew is one of the most powerful Atlantic hurricanes in recent history.
It is forecast to reach the Bahamas tomorrow and possibly reach Florida by Thursday as a major hurricane, though weaker than at present, the US National Hurricane Center (NHC) said.
Before that, it will hit Cuba.
"It has the potential of being catastrophic," said Dennis Feltgen, a meteorologist and spokesman for the Miami-based hurricane centre, when asked about Matthew's expected impact on Haiti.
Haitian Interior Minister Francois Anick Joseph said 30,000 people were in areas of risk who should be moved before the storm hits.
However, in Haiti's largest slum, the seaside Cite Soleil in capital Port-au-Prince, Mayor Frederic Hislain said 150,000 people whose homes he said were threatened needed to be bused to safer places.
"Those people are living all along the seashore in a bunch of huts which usually can't even really protect them from ordinary rain. Now we are talking about a strong hurricane. Imagine the disaster we may have to face here."
Many people are reluctant to leave their homes due to fears their belonging will be stolen.
Matthew was about 225km south of western Haiti at 2100GMT, the US hurricane centre said.
It is expected to bring 38-101cm of rain to parts of Haiti and the Dominican Republic.
Crawling north at just 11km/h, the storm threatens to linger enough for its winds and rain to cause great damage.
Haiti is prone to flash floods and mudslides because most of its hillsides have been stripped bare by people cutting down trees to make charcoal to sell for fuel.
