Unending images on television images showing Americans dehydrated, hungry and unable to escape an uninhabitable New Orleans shocked the richest nation on earth as a desperate Third World nation in trouble.

Americans humbled by failures in Iraq saw that the authorities could not quickly cope with a natural disaster at home, The New York Times said Friday as thousands were feared dead in the costliest natural disaster in US history.

"The response you'd expect from an advanced country never happened," said Paul Krugman, a highly reputed economist from Princeton University.

"After 9/11, hard questions were deferred in the name of national unity, then buried under a thick coat of whitewash. This time, we need accountability," he said.

Disaster planners were well aware that New Orleans could be flooded by the combined effects of a hurricane and broken levees, yet somehow the government was unable to immediately rise to the occasion.

Weathermen had also given ample warnings before Hurricane Katrina struck, in sharp contrast to the absence of any warning when terrorists turned hijacked airplanes into the most lethal acts ever carried out in the United States in 2001.

"If this is what happens when the nation has two days of advance warning, imagine the aftermath of a surprise attack using a chemical, biological or nuclear device," said Paul Light of the non-partisan Brookings Institution.

Ironically, he said, a Category 5 hurricane was already on the Department of Homeland Security's list of 15 planning scenarios for emergency response.

The near helpless state of the authorities was evident in nearly every other department as Katrina wrought havoc in New Orleans, which bore the brunt of the hurricane in the US Gulf coast.

Up to 20,000 people who heeded warnings to evacuate to the Superdome stadium were made to spend days in sweltering squalor at the site of some of the most chaotic scenes.