As US Army engineers started to pump out the floodwaters after repairing a major breach in the city's flood levee, Mayor Ray Nagin said: "It’s going to be awful and it’s going to wake the nation up again."

Mr Nagin, who believes that the death toll from the disaster could top 10,000 in New Orleans alone, told NBC's Today programme that it would take three weeks to remove the water and another few weeks to clear the debris. It could also take up to eight weeks to get the electricity back on. "I’ve gone from anger to despair to seeing us turn the corner," he said.

But the floodwater being pumped into Lake Pontchartrain will be filled with chemicals, petrol, industrial waste and sewage, said Ivor van Heerden, head of a Louisiana State University centre that studies the public health impacts of hurricanes.

He said that petrol, anti-freeze, bleach, human waste, acids, alcohols and a host of other substances must be washed out of homes, factories, refineries, hospitals and other buildings.

“I don’t see how we could treat all that water,” Mr Mallett said. The result could be a second wave of disaster for southern Louisiana, said Harold Zeliger, a Florida-based chemical toxicologist and water quality consultant. “In effect, it’s going to kill everything in those waters,” he said.

Survivors who remain in New Orleans are also faced with the threat of fire. Firefighters said that because there was no electricity, people were using candles for light in the old, wooden buildings that make up many New Orleans neighbourhoods.

In the historic Garden District, home to some of the city’s oldest wooden structures, two houses disappeared in a roaring blaze.

Soldiers from the US Border Control force called for emergency services and evacuated neighbouring buildings, but with no water pressure in hydrants and communications in turmoil, there was little they could do to stop the fires. Helicopters are now being deployed to drop water on one blaze.

“Of course we’re using candles. What else we gonna do? We got no electricity,” said Junior Jones, 71, whose house in central New Orleans was on fire. He said he had not evacuated. “I’m sick, in a wheelchair. I could hardly walk. Where am I going to go?”

In Washington, where Congress resumed after a five-week break, President Bush deflected criticism about his own handling of the crisis by announcing that he would oversee an investigation into what went wrong and why - in part to be sure the country could withstand more storms or attack.

"Bureaucracy is not going to stand in the way of getting the job done for the people," the president said after a Cabinet meeting at the White House. "What I intend to do is lead an investigation to find out what went right and what went wrong."

Mr Bush added: "We still live in an unsettled world. We want to make sure we can respond properly if there is a WMD (weapons of mass destruction) attack or another major storm."

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