"Powerful, category-four Hurricane Isabel is getting a little better organized as it moves westward," the service said, adding: "Some slight strengthening is possible during the next 24 hours."

Isabel was downgraded from a maximum-intensity category five to category four on the Saffir-Simpson hurricane scale, after its wind speeds dropped slightly from 260 kilometers (160 miles) per hour to 240 kilometersmiles) per hour.

At 1500 GMT, Isabel was located about 650 kilometers (405 miles) northeast of San Juan, Puerto Rico and moving west at a rate of about 17 kilometersmiles) per hour, forecasters said.

Isabel was expected to continue on that trajectory over the next 24 hours. "Large ocean swells and dangerous surf conditions are likely over portions of the Leeward Islands, the Virgin Islands and Puerto Rico over the next several days," the weather service said.

The US State Department on Friday authorized its non-essential diplomats to leave the Bahamas and warned US citizens against visiting the islands because of the damage that could be caused by Hurricane Isabel.

In addition, the department advised US citizens already in the Bahamas to consider leaving as the full-strength hurricane headed westward toward the central Antilles.

"The US State Department has given permission for non-emergency US personnel and family members of US personnel in the Bahamas to depart the Bahamas voluntarily," it said in a statement.

US citizens should "avoid travel to the Bahamas at this time due to the threat posed by Hurricane Isabel," it said, adding: "US citizens in the Bahamas should consider departing until the storm has passed."

The last Atlantic storm to reach maximum intensity was Hurricane Mitch, in

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