Nagin also called for an all-out evacuation of the city's remaining residents. Asked how many people died, he said: "Minimum, hundreds. Most likely, thousands."

With most of the city under water, Army engineers struggled to plug New Orleans' breached levees with giant sandbags and concrete barriers, and authorities drew up plans to clear out the tens of thousands of remaining people and practically abandon the below-sea-level city.

Nagin said there will be a "total evacuation of the city. We have to. The city will not be functional for two or three months." And he said people would not be allowed back into their homes for at least a month or two.

If the mayor's death-toll estimate holds true, it would make Katrina the worst natural disaster in the United States since at least the 1906 San Francisco earthquake and fire, which have blamed for anywhere from about 500 to 6,000 deaths. Katrina would also be the nation's deadliest hurricane since 1900, when a storm in Galveston, Texas, killed between 6,000 and 12,000 people.