A man crosses a street in New Orleans during Hurricane Isaac, Aug. 29, 2012. (David J. Phillip/AP)

A storm surge from Hurricane Isaac topped a levee in Plaquemines Parish south of New Orleans early Wednesday, officials said, trapping those who chose not to evacuate.

Plaquemines Parish President Billy Nungesser said the 18-mile-long, 8-foot-high levee—which is not part of the nearly $15 billion federal levee system constructed after Hurricane Katrina—was in the process of being raised.

"We knew we had a potential storm surge of 9 to 12 feet—we had an 8-foot levee," Nungesser said on CNN. "We're trying to get the few people who have stayed out. We've got a serious situation over there."

[Live-blog: Hurricane Isaac batters New Orleans, Gulf Coast]

Isaac made landfall at 6:45 p.m. CT Tuesday in Plaquemines Parish, and the slow-moving Category 1 hurricane—now centered about 50 miles southwest of New Orleans with maximum sustained winds topping 80 mph—is expected to dump as much as 20 inches of rain in several parts of Louisiana.

"Not only did we see the worst-case scenario, it got worse than that by this storm just stalling," Nungesser said. "So the levees can only take so much."

Nungesser said there were reports of up to 12 feet of water in some homes. "This is something I've never seen before," he said. "And I rode out Katrina."

[Slideshow: Hurricane Isaac pounds Gulf Coast]

Nungesser said three parish residents, including a woman on a roof, were saved by a private boat. Rescue workers were waiting for conditions to improve—and skies to lighten—before attempting other rescues.

"We're working with the U.S. Coast Guard to rescue people stranded on top of the levee," Nungesser said at a press conference.

Part of Plaquemines Parish was under a mandatory evacuation order, though it's not clear how many of its 26,000 residents left before the storm.

"There are homes inundated and some folks trapped by water in those homes," Guy Laigast, director of homeland security for Plaquemines Parish, told the Weather Channel.

[Related: Hurricane Isaac as seen from space]

"Over 150 people have had to be rescued from #Isaac flooding," CNN's Rob Marciano tweeted. "The majority were within mandatory evacuation zones."

According to News Orleans' Times-Picayune, Jesse Schaffer and his son have been rescuing stranded residents with their boat.

"We've rescued at least 23 people including children," Jesse Shaffer Jr. said.

The Army Corps of Engineers said the New Orleans levee protection system appeared to be working. Meanwhile, more than 500,000 customers were left without power in Alabama, Louisiana and Mississippi, utility companies said. According to The Associated Press, most of the outages are in areas around New Orleans.

A tornado warning has also been issued in southern Mississippi.