Isabel had lost power as it churned the Atlantic overnight and the US National Weather Service downgraded it to a category four hurricane on the Saffir-Simpson scale.

Air Force and Hurricane Hunter airplanes clocked winds of 157 kilometers (98 miles) per hour, the service said. Instruments measured the higher speeds just above the surface of the Atlantic.

At 2100 GMT, Isabel was located about 600 kilometers (375 miles) northeast of San Juan, Puerto Rico and was moving west at a rate of about 19 kilometers (12 miles) 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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