The Federal Emergency Management Agency (FEMA) scrambled to activate its regional center in Atlanta, Georgia to help local authorities in the event of massive evacuations.

"One of our priorities is identifying shelter alternatives and temporary housing," explained FEMA spokesman Mike Widomski.

Experts say landfall might occur anywhere over a wide stretch of the US east coast, from the southern state of Georgia to the New England region, north of New York.

With hurricane force winds extending out up to 185 km (115 miles), the US Navy was debating whether to send the Atlantic Fleet out to sea to ride out the storm.

About 70 US navy ships, including the aircraft carrier USS Ronald Reagan, were in southeastern US ports threatened by Hurricane Isabel.

"What they're doing is looking at the storm's track, looking at the storm's speed, trying to see what decision makes the most sense," said Ted Brown, an Atlantic Fleet spokesman at Norfolk, Virginia.

A decision on whether to move the fleet from port was expected later Monday, he said.

The last time the Atlantic Fleet scattered from its main base in Norfolk to avoid a storm was in 1999, he said.

"It's very expensive to get that many ships under way," Brown said.

"We don't want to make the wrong decision, but obviously these ships are much more capable of riding out a storm at sea where they can maneuver around the path of storm than they are sitting at pier where they can do considerable damage," he said.

Not all 70 ships now in port would be able to get underway because some of them are undergoing repair in shipyards, he said.

"Any ship that can get underway, if the order comes to sortie, will get under way," he said.

The storm was on a slow west-northwestern track Monday, easing a shade but remaining a powerful potential threat for the US east coast later in the week, forecasters said.

At 1500 GMT, the center of Isabel -- now a Category Four storm on the Saffir-Simpson scale -- was about 780 miles (1255 km) south-southeast of Cape Hatteras, North Carolina, or about 505 miles (815 km) east of Nassau, the Bahamas, the National Hurricane Center said.

The storm was moving toward the west-northwest near 13 kmh (8 mph) and a turn to the northwest is expected over the next 24 hours, it added.

"Satellite imagery indicates that the inner core structure continues to break down ... with a less well-defined eye in visible imagery" it explained, but "Isabel could still be a major hurricane at landfall."

"It has weakened but that's very relative," said Scott Kiser, National Weather Service headquarters, in Silver Spring, Maryland, adding that if even if the storm drops to category three before landfall, it would still be "a major storm, very threatening."

Maximum sustained winds were near 220 km/hr (140 mph) with higher gusts. "Although fluctuations in intensity are common in major hurricanes little overall change in strength is forecast during the next 24 hours," it added.

"Large ocean swells and dangerous surf conditions are likely over portions of the Greater and Lesser Antilles ... The Turks and Caicos Islands and the islands of the Bahamas over the next few days.

"These dangerous surf conditions will also affect portions of the southeastern US coast during the next several days," the center warned.

Category Five hurricanes can cause extensive damage on land: roofs ripped from homes, entire small buildings blown over or away, major damage to lower floors of buildings near the shoreline and shrubs, and trees and signs ripped down, according to the National Hurricane Center.

Category Four storms have been known to take roofs off smaller homes, seriously damage buildings near the shore, erode beaches and cause widespread flooding.

TERRA.WIRE