He urged residents in threatened areas to get ready for possible evacuation to safety.

The threat caused authorities to order visitors on the Florida Keys chain of islands to head the US mainland. It also prompted NASA to postpone Tuesday's planned blast-off of Atlantis and make preparations to return the space shuttle from the launchpad to its hangar.

Residents in the Keys started boarding up their homes and businesses, and stocking up on key items, as authorities opened emergency shelters.

Forecasters warned that Ernesto could draw fuel from the warm Straits of Florida waters after it leaves Cuba, and could hit Florida as a full-blown hurricane late Tuesday or early Wednesday.

The tropical storm dumped rain on eastern Cuba Monday, raising fears of flashfloods and mudslides, but was far weaker than originally anticipated, with maximum sustained winds of 64 kilometers (40 miles) per hour at 1500 GMT Monday.

Its eventual strength will depend largely on how long it remains over Cuba and where exactly it heads back to sea, according to the Miami-based National Hurricane Center (NHC).

Forecasters at the center said that under certain conditions Ernesto could thunder ashore in Florida at category three, the same level Hurricane Katrina had reached when it devastated New Orleans and other parts of the US Gulf Coast one year ago.

A category three hurricane packs winds of between 178 and 209 kilometers (111-130 miles) per hour.

The storm's projected track indicates Ernesto would sweep over the northern Florida Keys late Tuesday or early Wednesday, slam ashore just south of Miami a few hours later and emerge back in the Atlantic off northeastern Florida on Thursday.

NHC forecasters did not rule out a deviation from the anticipated track, and projections show that almost all of Florida is at risk.

Ernesto became this year's first Atlantic hurricane on Sunday morning, but was downgraded to a tropical storm later in the day.

The storm dumped torrential rains on Haiti as it passed the Caribbean country's southwestern tip before hitting Cuba early Monday.

Fearing a far more powerful storm, Cuba had ordered the evacuation of 600,000 people from threatened areas. All domestic flights were suspended.

In the past, Cuban President Fidel Castro had personally directed emergency operations when a major storm hit the island, but the 80-year-old communist leader is now recovering from intestinal surgery and has temporarily ceded power -- for the first time since 1959 -- to his brother Raul.

A hurricane watch was also declared for several of the Bahamas islands.

Related Links
US National Hurricane Center