Hurricane Isaac bore down on New Orleans early on Wednesday after making landfall, seven years to the day after Katrina devastated the 'Big Easy' and killed hundreds along the US Gulf Coast.

Isaac, which reached hurricane strength on Tuesday and was packing maximum sustained winds of 80 miles (130 kilometers) per hour, lashed southern Louisiana with heavy rains and strong winds, as residents hunkered down.

The US National Hurricane Center said the category one storm had generated a 'dangerous storm surge' along the northern Gulf Coast, with a surge of 11 feet (three meters) reported in Louisiana and fears of coastal flooding.

Powerful winds knocked over trees and ripped down power lines, leaving some 390,000 people without power, according to Entergy Louisiana, a local utility.

More than 4,000 members of the Louisiana National Guard have been mobilized, with 48 boat teams deployed around New Orleans, according to the office of governor Bobby Jindal, who had warned residents to prepare for the worst.

New Orleans Mayor Mitch Landrieu said on Tuesday that the city could expect up to 16 inches (40 centimeters) of rain or more from the slow-moving hurricane.

"We have dodged a bullet in the sense that this is not a category three storm," he said, "But a category one at this strength... is plenty big enough to put a big hurt on you if you fall into complacency. Let's not do that."

US President Barack Obama urged people to take the threat seriously, warning of the possibility of major flooding and damage.

"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," Obama said, adding: "Now is not the time to tempt fate."

Obama said he had managed a wide-ranging effort by federal and local governments to prepare for the storm.

His televised appearance showed the power of an incumbent to intervene at politically advantageous moments, just as Republicans met in Florida to nominate Mitt Romney as their candidate for the November presidential election.

Rising waters from Isaac meanwhile spilled over a levee south of New Orleans and inundated a residential area that had been ordered evacuated, a local official said early on Wednesday.

The flooding in Plaquemines Parish, part of a tongue of land extending into the Gulf of Mexico south of New Orleans, saw waters rise 12 feet (3.6 meters) in some homes, Parish President Billy Nungesser told CNN.

"As that water flows over the top, it eventually will eat out portions of that levee, which then it washes away," he said.

Nungesser said at least half of the 2,000 people living in the threatened area had left ahead of the storm.

A hurricane warning remained in effect for metropolitan New Orleans, a city known as the Big Easy for its jazz and easy-going lifestyle.

As of 1100 GMT Wednesday, the eye of the storm was about 50 miles (75 kilometers) southeast of New Orleans, moving inland, the center said.

Jindal said Tuesday that his state had contacted Washington about getting reimbursed for hurricane-preparation spending -- an allusion to agonizing delays in getting federal help after Katrina blasted the city.

While most New Orleans residents heeded calls to hunker down in their homes, a steady stream of more adventurous souls headed Tuesday to the banks of Lake Pontchartrain to feel the power of the wind and watch the crashing waves.

Other die-hards spilled into the handful of bars still open in the famed French Quarter, but the streets emptied as heavier rains and darkness fell.

The timing of the storm -- set to bear down on New Orleans on the seventh anniversary of Katrina - nevertheless had many here on edge.

"It brings back a whole lot of memories," said Melody Barkum, 56, who spent days stranded on a roof without food or water after Katrina struck. "I'm not afraid. If I can survive Katrina, I can survive this."

Katrina left behind a devastating sprawl of destruction and death when it hit New Orleans on August 29, 2005, and a bungled response by the Bush administration tarnished the president's second term in office.

Some 1,800 people were killed along the US Gulf Coast and in New Orleans thousands were left stranded on the roofs of their houses for days after Katrina's storm surge smashed levees long-warned to be inadequate.

Those who made it to dry land faced deadly violence and looting as the city descended into chaos and officials failed to provide water and food - let alone security and medical aid - in the sweltering heat.

Officials insisted billions of dollars spent to reinforce the city's storm levees and pumps will protect New Orleans from flooding this time, and Isaac is nowhere near Katrina's strength.

Mandatory evacuations were ordered in a number of coastal counties in Louisiana and Alabama, where people typically build their homes on stilts.

The slow-moving and massive storm could dump as much as 20 inches (50 centimeters) of rain on isolated areas and spawn tornadoes, the NHC warned.