Texas faces "potentially catastrophic" damage when Hurricane Ike hits in the next few hours, US Homeland Security Secretary Michael Chertoff has warned.
He said as many as 100,000 homes could be destroyed by flooding.
There are fears the hurricane could cause a high storm surge that leaves miles of low-lying coast underwater.
Residents of the Texan coastal city of Galveston face "certain death" if they do not evacuate, the National Weather Service has warned.
At 1800 GMT, the unusually large storm was about 165 miles 'Certain death' warning over Ike ...
Hurricane Ike to strengthen before hitting Texas ...
Hurricane Ike weakens over Cuba ...
Judge rejects 9/11 burial claims ... (270km) south-east of Galveston, with winds above 105 mph (165km/h).
The massive system is already buffeting Texas and causing flooding along the Louisiana coast, still recovering from last weekend's Hurricane Gustav.
More than a million people in Texas have been advised to leave their homes before Ike hits late on Friday or early Saturday morning.
But people in Houston city have been told to shelter at home, board up their properties and stockpile supplies.
Authorities are trying to avoid a repeat of 2005, when some 110 people in Houston died during a chaotic evacuation in the face of Hurricane Rita.
The US National Hurricane Center said Ike could grow from a Category Two to a Category Three storm - a "major hurricane" - by the time it reaches the coast.
Ike has already caused devastation in Cuba and Haiti, where hundreds of people have died in several tropical storms over the last month.
The hurricane's predicted path will take it through Galveston and on to Houston, home to America's biggest oil refinery and Nasa's Johnson Space Center.
"Our nation is facing what is by any means a potentially catastrophic hurricane," said Mr Chertoff.
He added: "This storm is so big in fact that its impact is already being felt all along the Gulf Coast."
The Texas authorities have laid on more than 1,000 buses to facilitate the mass evacuation.
In Galveston - scene of the country's deadliest hurricane, which killed at least 6,000 people in 1900 - 75 buses are taking residents to the state capital, Austin.
Weak and chronically ill hospital patients are also being moved.
But some residents were ignoring the dire warnings in Galveston, where Ike threatens to bring a 20ft (7m) high storm surge.
"If it ain't your time you ain't going anywhere," Emory Sallie, 44, told AP news agency, while walking and drinking a beer near his home.
Ports are shut and almost all energy production in the Gulf of Mexico has been suspended as a precaution, although Ike was expected to miss most of the installations.
The storm would be the first major hurricane to hit a US metropolitan area since Katrina devastated New Orleans three years ago.
President George W Bush has declared a federal emergency in Texas, allowing funds to be freed to help the state deal with the storm.
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(BBC)
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