"We probably have 80 percent of our city under water; with some sections of our city the water is as deep as 20 feet (7 metres). Both airports are underwater," Mayor Ray Nagin said in a radio interview.

Nagin added he expected the death toll to be significant.

"This is catastrophic," Louisiana Gov. Kathleen Blanco said.

She said a plan was being developed to evacuate tens of thousands of people in the Superdome -- the city's covered football stadium -- and other shelters.

"The first goal is to bring enough supplies in to sustain those folks until we can develop a network to get them out," Blanco told a news conference in the Louisiana state capital, Baton Rouge. "There's no power. It's hot. it's difficult to get food to them. There's water lapping at the foot of the Superdome now."

"We know many lives have been lost," Blanco added. "Those numbers will be forthcoming as the process clarifies, but these are very difficult times for Louisiana."

'STATE OF DEVASTATION'

New Orleans, a city that usually throbs with carnivals and the sound of jazz and blues, was in a "state of devastation," said Nagin.

Looting reportedly was widespread. Some thieves were seen taking necessities, while others grabbed clothing, athletic shoes and jewelry from shops in the tourist districts.

Smoke columns dotted the cityscape as fires burned out of control because of a lack of water pressure and manpower. Most of the area lacked potable water, electricity and reliable communications equipment. Natural gas leaks were reported.

Fears grew about pollution, with floodwaters believed to be carrying sewage, spilled fuel and other pollutants from residential and commercial districts inundated in the flood.

Natural dangers emerged, too. Authorities reported spotting a 3-foot (0.9-metre) shark cruising the city.

In many residential areas, TV pictures showed brackish water up to roof level after the surge caused by Katrina breached at least two sections of protective levees that hold out Lake Ponchartrain.

The US military was planning to drop giant sandbags loaded with gravel to repair a major break in a protective levee that caused the flooding of most of New Orleans, a spokesman for the Army Corps of Engineers said on Tuesday.

The main puncture was 500 feet (150 metres) long, said Mike Lambert, spokesman for the Louisiana Department of Transportation.

Much of New Orleans lies in a bowl below sea level, bounded by the lake and the Mississippi River, North America's biggest river, which curves along the south of the city before discharging in the Gulf of Mexico.

"We always were afraid the bowl that is New Orleans would fill quickly," Walter Maestri, emergency management coordinator for Jefferson Parish, said in a radio interview. "Now with the water rising today, it appears to be filling slowly," he said.

"The water is rising so fast I cannot begin to describe how quickly it's rising," Tulane University Medical Center Vice President Karen Troyer-Caraway told CNN. "We have whitecaps on Canal Street, the water is moving so fast."

The downtown hospital was surrounded by 6 feet (2 metres) of water and considering evacuating its 1,000 patients.

Weather experts had predicted the city would be overwhelmed by the impact of Katrina, which tore across the Gulf coast on Monday, but initially damage appeared less than catastrophic.