Wilma was dumping rain on Central America and Mexico, and forecasters warned on Wednesday of a "significant threat" to Florida by the weekend.
The storm's power multiplied greatly over the past day. On Tuesday morning Wilma grew from a tropical storm into a weak hurricane with 129kph winds.
At 2.30am (0630 GMT) on Wednesday, US Air Force reconnaissance planes measured Wilma's top sustained winds at 282kph, making it a Category 5 hurricane, according to the National Hurricane Centre in Miami.
Two hours earlier, Wilma had been declared a Category 4 storm with maximum sustained winds of 241kph.
Wilma's pressure readings on Wednesday morning indicated that it was the strongest hurricane of the season, said Trisha Wallace, a meteorologist at the National Hurricane Centre in Miami.
Wilma had a reading of 892 millibars, the same reading as a devastating unnamed hurricane that hit the Florida Keys in 1935.
"We do not know how long it will maintain this Category 5 state," Wallace said.
"It does look like it poses a significant threat to Florida by the weekend. Of course, these are four- and five-day forecasts, so things can change"
Dan Brown, meteorologist at the US National Hurricane Centre
Jamaica, Cuba, Nicaragua and Honduras were getting heavy rain from the storm, though it was not likely to make landfall in any of those countries, she said.
Channel between Cuba and Mexico's Cancun region - then move into the storm-weary Gulf.
Forecasts indicated it would probably turn towards the narrow Yucatan
Threat to Florida
"It does look like it poses a significant threat to Florida by the weekend. Of course, these are four- and five-day forecasts, so things can change," said Dan Brown, a meteorologist at the US National Hurricane Centre.
With heavy rain, high winds and rough surf pounding coastal areas, flood-prone Honduras warned that Wilma posed "an imminent threat to life and property" and closed two seaports on its Caribbean coast.
Neighbouring Nicaragua and the Cayman Islands also declared an alert.
Honduras and its neighbours are recovering from flooding and mudslides caused earlier this month from storms related to Hurricane Stan.
At least 796 people were killed, most of them in Guatemala, with many more still missing.
Cuba issued a hurricane watch for the western end of the island from Matanzas to Pinar del Rio, as well as the Isle of Youth.
Mexico issued a hurricane watch for nearly all of its Caribbean coast from Punta Gruesa to Cabo Catoche, an area that includes the resort of Cancun.
Wilma has already been blamed for one death in Jamaica as a tropical depression on Sunday.
It flooded several low-lying communities and triggered mudslides that blocked roads and damaged several homes, said Barbara Carby, head of Jamaica's emergency management office.
She added that about 250 people were in shelters throughout the island.
Although the storm was not expected to approach Florida until the weekend, some residents have begun to buy water, canned food and other emergency supplies.
Many said they took every storm seriously now, after witnessing the devastation from a succession of hurricanes that had ravaged the southern United States.
"People have learned their lesson and know better how to prepare. We're not waiting until the last minute anymore," said Andrea Yerger, 48, of Port Charlotte, Florida.
"There's no scenario now that takes it toward Louisiana or Mississippi, but that could change"
Max Mayfield, director,
National Hurricane Centre
She was buying material to protect her house, which had to be gutted because of extensive damage from Hurricane Charley last year.
Wilma's track could take it near Punta Gorda on Florida's southwestern Gulf Coast and other areas in the state hit by Charley, a Category 4 storm, in August 2004; forecasters urged Florida residents to closely monitor Wilma.
In the Cayman Islands, the authorities urged businesses to close early on Tuesday to give employees time to prepare for the storm. Schools were ordered to close on Wednesday.
In Mexico, the MTV Latin America Video Music Awards ceremony, originally scheduled to be held on Thursday at a seaside park south of Cancun, was moved up one day to avoid possible effects from Wilma.
Out of names
Forecasters said Wilma should avoid the central US Gulf coast devastated by hurricanes Katrina and Rita earlier this year, which killed more than 1200 people and caused billions of dollars in damage.
"There's no scenario now that takes it toward Louisiana or Mississippi, but that could change," said Max Mayfield, director of the National Hurricane Centre.
The storm is the record-tying 12th hurricane of the season, the same number reached in 1969; 12 is the most in one season since record-keeping began in 1851.
Wilma has also exhausted the list of names for storms this year. The six-month hurricane season does not end until 30 November. Any new storms would be named with letters from the Greek alphabet, starting with Alpha.