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By Randall Mikkelsen

WASHINGTON, Aug 28 (Reuters) - Poor areas of New Orleans hardest hit by Hurricane Katrina remain vulnerable to flooding because flood control improvements have not been completed, a U.S. official said on Thursday as Tropical Storm Gustav threatened a possible new strike on the city.

Gustav hit Jamaica on Thursday and was expected to strength in coming days and head toward the U.S. Gulf Coast, possibly threatening New Orleans.

Improvements in flood control have left the New Orleans area better-protected overall than when Katrina devastated it three years ago, and the government is better prepared to handle a major storm, disaster officials said in a telephone news conference.

The U.S. Army Corps of Engineers was also rushing to install temporary flood-prevention structures along the city's Inner Harbor Navigation Channel.

But the improvements are years from being complete and several neighborhoods remain vulnerable, said program director Bill Irwin of the U.S. Army Corps of Engineers.

"There are those high-level risk areas -- such as the Lower 9th Ward, Gentilly, St. Bernard, New Orleans East and the West Bank -- still," Irwin said. "There are gaps in the system."

The low-income Lower 9th Ward was broadly devastated by Katrina and became a symbol of the storm's human impact. Its rebuilding has been slow.

Irwin declined to say whether the areas he mentioned were more or less vulnerable than before Katrina, but he said the flood-control improvements remained a "work in progress" that would not be complete until 2011.

"Overall it's the best flood-protection system that it (New Orleans) has ever had and it's much better overall than in Katrina," he said. "Some of these areas we haven't been able to get to, because it's a multiyear project," Irwin said.

"With this particular threat, there are those vulnerabilities that still exist," he said.

Gustav was expected to soon become a hurricane. New Orleans remained near the middle of the range of possible landfall locations in the U.S. Gulf Coast.

The Bush administration's response to Katrina was intensely criticized as insufficient, poorly organized and unprepared.

'WORKED VERY HARD'

Harvey Johnson, deputy administrator of the Federal Emergency Management Agency, said: "We've all worked very, very hard over the last two years in planning and training and assessing and exercising and being ready for the next big hurricane, and Gustav is going to be it."

The American Red Cross has set up field operations in Alabama, Mississippi, Louisiana and Texas to prepare shelters and evacuation plans, said vice president Armond Mascelli.

FEMA Administrator David Paulison was in Louisiana and U.S. Homeland Security Secretary Michael Chertoff was on his way.

National Guard Gen. Donald Fick said there were about 65,000 reservists available in areas threatened by the Gustav. The guard intended to keep them in position for Tropical Storm Hanna, which has the potential to hit Florida and the Bahamas next week.

The Army Corps intended to have the New Orleans Navigation Channel flood reinforcements in place along a 1,800-foot (549-metre) stretch by Monday, Irwin said.

The Coast Guard was on heightened alert, preparing areas such as the Houston Ship Channel and Port of New Orleans and relocating boats and planes, said Rear Adm. Brian Salerno.

Any threatened ports would close about 12 hours before an anticipated strike, and commercial operators would send ships out to sea to ride out the storm about 24-48 hours before a strike, Salerno said.

(Reporting by Randall Mikkelsen; Editing by Alan Elsner)

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