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The state of Texas has urged the US federal government to put 10,000 troops on standby for search and rescue work as Hurricane Rita approaches.
While the storm has weakened, the Gulf Coast as far west as Mexico is on alert and more than a million people are fleeing, causing huge traffic jams.
Texas is due to bear the brunt of Rita but storm rains also threaten New Orleans' newly repaired flood defences.
President George W Bush is due to visit Texas to see preparations there.
In other developments:
- A thousand state troopers and 5,000 National Guard are already making preparations for the storm in Texas
- National Guard trucks are taking badly-needed fuel to petrol stations and stranded motorists around Houston
- Oil companies are closing refineries in Texas and moving workers from offshore rigs.
Mr Bush will fly to Texas on Friday to "get a firsthand look at the preparations... and to show our support for the first responders as they get ready", said White House spokesman Scott McClellan.
The visit, Mr McClellan added, would be "as minimal a footprint as possible" and would not interfere with emergency planning in the president's home state.
Mr Bush, who was criticised for the federal government's response to Hurricane Katrina last month, met security chiefs in Washington and told reporters that "officials at every level of government" were "preparing for the worst".
According to the National Hurricane Center, Rita weakened on Thursday to a Category Four, with winds slowing to 145mph (230km/h) by late afternoon.
But it remained "extremely dangerous", being at least as powerful as Katrina and threatening 600km (370 miles) of coastline.
Governor Rick Perry phoned Mr Bush on Thursday morning to make his troop request "to assist with search and rescue efforts in the immediate aftermath of the storm".
The hurricane is due to make landfall late on Friday local time.
A mandatory evacuation order in the island city of Galveston was rapidly observed and 90% of the city's 57,000 residents had left by Thursday afternoon.
In Houston, a centre of the US oil industry, low-lying areas at risk from flooding were also emptying.
The post in Galveston is standing in letter-boxes, newspapers lie on driveways and the rubbish waits for collection, the BBC's Daniela Relph reports.
Petrol and food are scarce but the emergency procedures have been organised and orderly.
"It feels like a ghost town to me, and that's a good thing," Galveston City Manager Steven Leblanc told AFP news agency.
Almost everyone has headed north towards Houston and the roads are so clogged that a journey of normally an hour now takes around eight hours, our correspondent says.
To the north of Houston, a traffic queue more than 150km (93 miles) long was reported as people headed inland in high temperatures.
Having closely watched New Orleans and Louisiana, our correspondent says, Texas is determined to get its hurricane response right.
In neighbouring Louisiana, Governor Kathleen Blanco urged people to leave the south-western coast, already battered by Hurricane Katrina.
She estimated that between 300,000 and 500,000 people would go.
Engineers have been seeking to bolster the floodwalls in New Orleans, which were overcome by Katrina's storm surges.
Some estimates say that even a few inches of rain would overcome the flood defences.
"The concern with New Orleans right now is the rainfall and the weakened state of the levees," said Mr McClellan.
Mexico has not evacuated its coastal state of Tamaulipas where tropical storm conditions are predicted.
However, the state is on a "yellow alert" in case Rita should veer west, away from Texas.