Wilma, the record-equalling 21st storm of the Atlantic season, strengthened to the highest level on the SaffirSimpson scale, the Miami-based center said in a bulletin at 0630 GMT.

"Data from a reconnaissance aircraft indicate that Hurricane Wilma has become an extremely dangerous category five hurricane," the center said half an hour after it had announced the storm had strengthened to category four.

Winds at the center of the storm were clocked at 281 kilometers (175 miles) per hour, the center said, adding that the pressure, 892 millibars, was the lowest observed in 2005 -- and the lower the pressure, the stronger the storm.

The hurricane was located 270 kilometers (170 miles) south-southwest of Grand Cayman Islands and 640 kilometers (400 miles) southeast of Cozumel, Mexico, and moving slowly west-northwest toward Mexico's Yucatan peninsula, but was expected to turn toward the northwest within 24 hours.

Wilma is expected to move into the Gulf of Mexico, then veer northeast toward Florida.

"All interests in the Florida Keys -- the southernmost US islands -- and the Florida peninsula should closely monitor the progress of Wilma," the hurricane center said.

World oil prices dropped amid hopes that Wilma would not hit oil installations on the storm-weary US Gulf Coast.

Authorities in Cuba, Jamaica, Mexico, Nicaragua and the Cayman Islands have all issued hurricane alerts.

Cuba has ordered 5,000 people evacuated from flood-prone areas on the storm's course, and Mexico put the tourist zones of Yucatan on alert.

Honduras also ordered preparations for evacuations as heavy rain started falling. Widespread flooding was reported in Jamaica from rainfall sparked by the hurricane.

Organisers of the MTV Latin Awards brought their annual ceremony at the Mexican resort of Cancun forward a day to Wednesday because of the storm.

Wilma is the 12th full-blown hurricane of the Atlantic season, and a series of them have left thousands dead in Central America and on the US Gulf Coast.

Hurricane Katrina, which was at category four when it made landfall on August 29, killed more than 1,200 in the United States, and Hurricane Stan last week left more than 2,000 dead in Guatemala alone.

Florida has already been battered by hurricanes Dennis and Katrina this year, and the state's governor, Jeb Bush, brother of President George W. Bush, was downcast at the prospect of a new hit.

"Why us?" he said. "How does a storm take a sharp 90 degree turn?"

A climate study released Monday said the continental United States will face more extreme temperatures during the next century and worse rainfall along the Gulf Coast.

The study, published by the journal Proceedings of the National Academy of Sciences, warned that greenhouse gases will likely swell to twice their current levels by the end of the century.

It predicted that the southwestern United States could endure as much as a 500 percent increase in hot events, leaving less water for the growing population; that the Gulf Coast region would receive more rainfall in shorter time spans; and that summers in the northeast would be shorter and hotter.

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