The lumbering Category 1 hurricane was lashing the Gulf Coast, threatening to flood towns in Mississippi and Louisiana with a deluge of rain, storm surges of up to 3.7-metres and top sustained winds up to 120km/h.
With the storm nearly stalled, rainfall accumulations totalling as much as 50 cm in some areas were expected.
"The federal levee system … is fine," New Orleans Mayor Mitchell Landrieu told local radio.
"There are no risks. It is holding exactly as we expected it to and is performing exactly as it should. There are no people on rooftops from flooding that even approximates what happened during Katrina," Landrieu said.
Police and National Guard units, many wielding automatic assault rifles, patrolled the virtually empty downtown quarter of the port city, which normally hums with tourists drawn to its jazz bars, Creole cuisine and French colonial architecture.
Tree limbs and street signs littered the streets and power was cut intermittently throughout the city, but authorities reported no security problems.
"Thus far it's been pretty easy," said Captain Jeremy Falanga of the Louisiana National Guard, who was stationed with troopers in front of the city's convention centre. "Not many people are outside, it's pretty buttoned up."
In low-lying Plaquemines Parish, which stretches southeast from New Orleans, emergency officials reported the overtopping of an 2.4-metre high levee between the Braithwaite and White Ditch districts.
Plaquemines Parish President Billy Nungesser said about 2 000 residents of the area had been ordered to evacuate but only about half were confirmed to have gotten out before Isaac made landfall late on Tuesday.
Isaac was wobbling north-westward near 10km/h, a slow pace that increases the threat of rain-induced flooding.
"On the east bank right now, we have reports of people on their roofs and attics and 3.7-4.2-metres of water," Nungesser told CNN.
"This storm has delivered more of a punch than people thought," he added.
Plaquemines Parish is cut in two lengthwise by the Mississippi River as it flows to the Gulf of Mexico. Much of it lies outside the greater New Orleans levee system, and construction projects to bolster protection are not complete.
It was not immediately clear how many people may have been stranded in the area, as driving rain and hurricane-force winds prevented a full-scale search.
No reports of injuries
But Plaquemines Parish public information officer Caitlin Campbell said there were no reports of injuries or deaths.
"The back levee along that area has been overflowing with water since earlier today. Up to 3.7-metres of water has filled that community from the back levee to the Mississippi River," Campbell said in an email statement.
"Rescue efforts are now in progress. Local residents are rescuing other residents at this time," she said.
Retired US Army Lieutenant General Russel Honore, who helped restore calm to New Orleans after Katrina, credited the "Cajun Navy" for conducting initial search-and-rescue operations in Plaquemines Parish.
He was referring to local boatmen on the coastal peninsula, plucking neighbours from flooded homes to safety.
Isaac was the first test for the $14.5-billion flood defence system of walls, floodgates, levees and pumps built after Katrina's storm surge. Katrina left large parts of New Orleans swamped and killed 1 800 people in the costliest natural disaster in US history.
Hundreds in and around New Orleans drowned in 2005 and many survivors waited for days to be plucked from their rooftops by helicopters. New Orleans endured days of deadly disorder and widespread looting.
Not as strong as Katrina
While not nearly as strong as Katrina – a Category 3 hurricane when it slammed into New Orleans on August 29 2005 – Isaac was a threat that authorities had warned repeatedly about underestimating.
Timothy Doody, president of the Southeast Louisiana Flood Protection Authority, said the storm surge from Isaac had seen the Mississippi River rise 2.4-metres in New Orleans between early Tuesday and Wednesday.
"The water coming up into the Mississippi River appears to be more than forecast," he said.
The river, which had been extremely low due to the extended Midwest drought was at 0.9-metre before the storm and the level rose to 3.3-metre, Doody said.
Some 409 000 Louisiana customers of utility Entergy Corp were without power as of Wednesday morning, the company reported. It warned that it would be unable to begin restoring power until winds drop below 48km/h.
While most city residents had opted to weather Isaac in their homes that was no option for the homeless.
"It was wet last night," said Stanley Lancaster (50) who spent the night under an overhang by the city's convention centre. "I'm just going to sit and wait till it stops."
At 12pm EDT (4pm GMT), Isaac's top sustained winds had weakened slightly to 120km/h and the storm was centred about 75km southwest of New Orleans, the US National Hurricane Centre said. It said hurricane force winds extended outward for 97km from the storm's centre.
Isaac killed at least 23 people and caused significant flooding and damage in Haiti and the Dominican Republic before skirting the southern tip of Florida on Sunday and heading across the warm waters of the Gulf of Mexico.
It spared Tampa, Florida, where the Republican National Convention is being held. But it forced party leaders to reshuffle the schedule and tone down what some might have seen as excess celebration about Mitt Romney's presidential nomination as Gulf Coast residents faced danger.
Oil production in the US Gulf of Mexico nearly ground to a halt and ports and coastal refineries curtailed operations.
Energy companies along the Gulf Coast refining centre braced for the storm's impact by shuttering some plants and running others at reduced rates ahead of Isaac's landfall.
Intense hurricanes such as Katrina – which took out 4.5-million barrels per day of refining capacity at one point – have flooded refineries, keeping them closed for extended periods and reducing fuel supplies.
This time, though, the US department of energy estimated that only 12% of the Gulf Coast's refining capacity had gone offline. Louisiana usually processes more than 3-million barrels per day of crude into products like gasoline.
Perceptions that the area's oil facilities would not sustain major damage left international benchmark Brent crude little changed in Wednesday afternoon trading at about $112.40 a barrel.
David Zelinsky, a meteorologist at the Miami-based Hurricane Centre, cautioned that Isaac was not going away anytime soon.
"We expect it generally to continue moving very slowly through Louisiana today, even into tomorrow. Beyond that, as it begins to weaken we expect it to move into northern Louisiana late Thursday into Friday and then north into Arkansas, Texas, Oklahoma, that region after that," Zelinksy said.
"We want to stress to everyone that a lot of the effects can be felt well removed from the centre," he said. – Reuters