Hurricane Wilma, which has swelled into a dangerous Category Five storm, is the strongest hurricane ever recorded, the US National Hurricane Center says.
It says the storm's barometric pressure - a measure of its strength - was the lowest on record in the Atlantic basin.
Its winds of near 165mph (270km/h) and heavy rains are threatening Cuba, Mexico and the Cayman Islands.
At least 11 people have been killed in Haiti by floods and landslides associated with Wilma.
Forecasters say the hurricane presents a "significant threat" to Florida, which it is expected to hit this weekend.
Officials in the vulnerable Florida Keys island chain have ordered visitors and non-residents to leave.
Map of predicted storm path
The Central American states of Honduras and Nicaragua have also issued alerts, and are expecting tropical storm conditions within the next 36 hours.
The hurricane, which was classified as a tropical storm on Tuesday, is expected to dump inches of rain across the region.
In Jamaica, Wilma was blamed for one death after heavy rainfall flooded several low-lying communities, blocked roads and forced 100 people into shelters, according to local officials.
As of 1400 (1800 GMT), the centre of the hurricane was about 300 miles (480km) south-east of Cozumel, Mexico.
Wilma is moving at a speed of 7mph (11km/h) towards Mexico's Yucatan peninsula, but is expected to turn north-west in the next 24 hours.
The Mexican authorities have told tourists to evacuate high-risk areas along the coast near the holiday resort of Cancun, AFP news agency reports.
The strong winds have forced music television station, MTV, to postpone its Video Music Awards Latin America, which had been scheduled to take place in the resort of Playa del Carmen.
Wilma is bringing heavy rains to the region
The storm is the record-tying 12th hurricane of the season, the same number reached in 1969. It is the most for one season since record-keeping began in 1851.
Residents in the Florida Keys have already begun buying emergency supplies.
"People have learned their lesson and know better how to prepare. We're not waiting until the last minute anymore," Andrea Yerger, of Port Charlotte, told the Associated Press news agency.
She was buying material to protect her house, which had to be gutted after suffering extensive damage from Hurricane Charley last year.
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Wilma has raised concerns regarding oil production in the Gulf of Mexico.
The areas threatened by the new hurricane are still recovering from the devastation caused by hurricanes Katrina and Rita.
The authorities in New Orleans have told residents to be ready to evacuate, although Louisiana is not in Wilma's predicted path at present.
Hurricane Katrina killed nearly 1,200 people along the US Gulf Coast in August. Hundreds more died in Mexico and Central America when Hurricane Stan struck early this month.
* Some meteorologist was suggesting that these hurricanes weren't natural, and I was thinking that he was crazy but, Wilma< went from a cat 1 to a cat 5 in 12 hours & for a moment it hit 215mph< they said it had to be an error or something but ...