Texas is under a state of emergency.
Rick Perry, the governor of Texas, said: "It's a big storm. I cannot overemphasise the danger that is facing us."
"It's going to do some substantial damage. It's going to knock out power. It's going to cause massive flooding."
Hurricane Ike was heading almost directly towards Galveston, with Houston - the fourth biggest city in the US and a major hub of the oil industry - lying beyond.
Steve LeBlanc, the city manager of Galveston, said: "It appears the storm has taken a turn towards Galveston and I guess the bottom line effect that we're most concerned about are the tides."
The authorities in Harris County, which encompasses Houston, said evacuations of the city's most flood-prone areas - home to about a quarter of a million residents - began on Thursday.
|Ike was expected to miss most of the Gulf's off-shore oil platforms [AFP]
However, in a calculated risk aimed at avoiding chaos, the authorities instructed most people in Houston to "shelter in place".
Bill White, the mayor of Houston, said: "They are in a safer, better position if they stay where they are."
Three years ago, evacuation orders ahead of Hurricane Rita created panic and a mass exodus from the city.
That evacuation proved deadlier than the storm itself, leaving 110 people dead after traffic-jammed vehicles ran out of petrol and overheated, and at least one bus burst into flames.
Hurricane warnings for Ike, meanwhile, were in effect over a 650km stretch of coastline from south of Texas's Corpus Christi to Morgan City, Louisiana.
Tropical storm warnings extended south almost to Mexico and east to the Mississippi-Alabama state lines, including New Orleans.
The hurricane's predicted path was set to take it away from the majority of the 4,000 offshore platforms that produce about a quarter of the US oil supply.
Ike has so far left more than 100 people dead in its wake across the Caribbean. In Cuba, more than 27,000 homes were destroyed while the majority of the deaths were in Haiti.