Jamaicans rushed to supermarkets and schools closed as Hurricane Ivan swept towards the Caribbean island with 258 kph winds yesterday after killing at least 20 people on the tiny spice island of Grenada.

Jamaican Prime Minister P.J. Patterson urged Jamaica's 2.7 million people to get ready as Ivan grew to a monster Category 5 storm, the most intense on the hurricane scale.

"All persons should prepare to relocate to shelters as soon as the Office of Disaster Preparedness instructs them to," Mr Patterson told Jamaicans. "It is better for us to breathe a sigh of relief, than for us to say, 'If we had known'."

Hurricane-weary Floridians eyed the storm warily after the state was hit by two hurricanes, Charley and Frances, in the last month.

Emergency managers in the Florida Keys ordered a mandatory evacuation for visitors, a measure taken well in advance because thousands of tourists need time to move recreational vehicles and boats up the 160 km island chain, linked by a single road.

An evacuation of Keys' residents is to begin today.

The Cayman Islands, a tiny British colony and key offshore financial center in the northwestern Caribbean, issued a hurricane watch for its 43,000 people yesterday, telling them hurricane conditions were possible within 36 hours.

In Jamaica, some gas stations in the capital ran out of fuel and long lines formed at food stores. The government ordered schools closed yesterday and today and three universities shut their doors.

Diplomatic missions shut down and the government urged employers to close by noon yesterday.

"This one looks like a killer. If it follows the same path, a lot of us will die," Kingston resident Jefferson James said.

The storm revived memories of Hurricane Gilbert, which hit Jamaica in September 1988. It was a Category 5 hurricane that was one of the most powerful storms in the recorded history of the Atlantic basin.