Barack Obama warns residents in Isaac's path
US President Barack Obama has warned residents in the path of Tropical Storm Isaac they should not "tempt fate" and should heed evacuation warnings.
Mr Obama has declared an emergency in Louisiana, allowing federal funds to be released to local authorities.
The storm remains close to hurricane strength and is predicted to make landfall by Tuesday night.
Isaac is expected to hit New Orleans seven years after the much stronger Hurricane Katrina struck.
"As we prepare for Isaac to hit, I want to encourage all residents of the Gulf Coast to listen to your local officials and follow their directions - including if they tell you to evacuate," Mr Obama said on Tuesday.
Speaking from the White House, he added: "Now is not the time to tempt fate. Now is not the time to dismiss official warnings. You need to take this seriously."
Isaac has killed at least 24 people in Haiti and the Dominican Republic, and caused significant flooding and damage in the Caribbean.
Isaac was "nearly a hurricane", the National Hurricane Center (NHC) said at 10:00 CDT (15:00 GMT).
The centre of the storm was located about 80 miles south-east of the mouth of the Mississippi River and was moving towards the coast at about 10 mph (17km/h).
Maximum sustained wind speeds were near 70 mph, and the storm was still predicted to strengthen into a category one hurricane - the lowest on the five-point hurricane scale, with winds of up to 95 mph.
The NHC warned that a possible combined "storm surge" and high tide would cause flooding in coastal areas.
Water would potentially reach 6-12ft (1.8-3.7m) above ground in south-west Louisiana and Mississippi, 4-8ft in Alabama and 3-6ft in south-central Louisiana.
Isaac is also threatening heavy rainfall of as much as 20in (51cm) in isolated spots, and could spark possible tornadoes along the northern Gulf Coast.
Hurricane warnings are in place for a swathe of land 400 miles (645km) wide, from Morgan City in Louisiana to the Florida-Alabama state line.
Many in New Orleans are still scarred by the memory of Katrina, which killed more than 1,800 people.
Queues of cars have been heading for higher ground, leaving New Orleans for Baton Rouge.
The bowl-shaped city of New Orleans is particularly vulnerable to storms, with the centre of the city the furthest below sea-level.
Residents are hoping that billions of dollars spent on reinforcing flood defences that failed catastrophically in 2005 will hold this time.
Officials have not ordered any evacuations, telling residents to reinforce their homes and stock up on supplies instead.
"It's going to be all right," said New Orleans Mayor Mitch Landrieu.
But in Houma, a city south-west of New Orleans, Simon and Crystal Naquin took refuge in an auditorium-turned-shelter, saying they were afraid their camper could flood.
Mr Naquin said he used to ride out hurricanes when younger, but not since seeing the damage wrought by hurricanes Andrew, Katrina and Rita.
"Now that I got kids, I've seen too much to say, 'Stay,'" Mr Naquin told Associated Press news agency.
In low-lying Plaquemines Parish, much of which lies outside the New Orleans levee system, a local official told Reuters news agency he was "really worried about the storm surge" - adding that a few more years were needed before flood protections were fully completed.
Federal Emergency Management Agency Administrator Craig Fugate has warned that Isaac poses a threat to areas far beyond New Orleans.
"This is a Gulf Coast storm. Some of the heaviest impact may be in Alabama and Mississippi," he said earlier.
Evacuations have been ordered for some low-lying Louisiana parishes and parts of coastal Alabama.
US offshore oil production is expected to be badly hit, as are refineries in lowland Louisiana.
As much as 78% of the Gulf's crude oil production and 48% of its natural gas production had been closed ahead of the storm, government figures showed.
BP and Chevron have shut down production, and BP is evacuating its platform.