Hundreds of thousands of people are evacuating their homes along the southern US coast as Hurricane Ivan heads inland from the Gulf of Mexico.
New Orleans could be devastated by the storm
A state of emergency has been declared in Florida, Louisiana and Alabama and nearly two million people have been urged to flee to higher ground.
Major roads out are jammed with traffic and ports and airports have closed.
At the same time, the Caribbean is on renewed alert as Tropical Storm Jeanne threatens Puerto Rico.
Hurricane warnings are in effect in Puerto Rico, the US Virgin Islands and parts of the Dominican Republic as the latest storm approaches.
Hurricane Ivan is expected to hit the US mainland on Thursday after spreading devastation across the Caribbean for a week.
At 1800 GMT, Ivan's centre was about 170 miles (272km) south of the Alabama coastline and moving northwards at 14mph (22km/h).
Forecasters say Ivan continues to be an "extremely dangerous" category four hurricane, with maximum sustained winds of 135mph (216km/h).
The US National Hurricane Center predicts that Ivan will make landfall as category three or higher, with hurricane force winds likely to extend up to 100 miles (160km) inland.
The exact place where Ivan will hit the US is likely to change in the coming hours.
The BBC's Daniela Relph in Alabama says there is a real sense of unease and even fear among many people living in the area.
The state of emergency is in force along a 300-mile (480-km) swathe from Louisiana to the Florida panhandle.
"I beg people on the coast: do not ride this storm out," said Mississippi Governor Haley Barbour.
New Orleans in Louisiana - the largest US city below sea level - is particularly vulnerable.
The BBC's Michael Buchanan in Washington says experts are warning that a direct hit on New Orleans could devastate the city for weeks.
Interstate 10, the main route out of New Orleans, is clogged with traffic, while only emergency vehicles are heading into Ivan's predicted path.
"Hopefully the house will still be there when we get back," New Orleans resident Tara Chandra told the Associated Press news agency as he packed up his car.
In 1965 Hurricane Betsy, a weaker storm than Ivan, was blamed for 74 deaths there.
Florida, which is still mopping up from two hurricanes over the past month, was bracing itself again.
"If we get the kind of tidal surge they are saying, the fishing boats are all going to be in the trees," bar owner Jamee Lowry of Perdido Key, Florida, told AP.
Oil prices rose nearly $1.50 a barrel on Monday as oil and
natural gas producers evacuated rigs in the Gulf of Mexico.
Meanwhile, the clear-up operation in areas already hit by Ivan continued.
There was a mass evacuation of western areas of Cuba, but the island escaped with minimal damage after the centre of the storm barely touched land.
An overnight curfew remains in effect in the Cayman Islands, where the hurricane wrought havoc on Sunday.
Flood waters are now receding on Grand Cayman island, the Caymans' representative in London told the BBC's Caribbean Service on Wednesday, while dozens of people are still missing in the British dependency, according to reports.
Ivan killed about 70 people in the Caribbean.
PREDICTED PATH OF HURRICANE IVAN