President Bush called for a "zero tolerance" policy against looters and profiteering today as New Orleans descended into lawlessness.

Three days after Hurricane Katrina slammed into the US Gulf Coast, armed gangs are roaming virtually unchecked through the flooded southern city, diverting police from the vital task of rescuing tens of thousands of residents trapped without food, power or fresh water.

Officials were even forced to suspend the evacuation of almost 25,000 flood refugees from the New Orleans Superdome after shots were fired at Chinook army helicopters overseeing the loading of people onto a fleet of prison buses.

In a television interview this morning Mr Bush defended himself from charges that his own response to the crisis was tardy - he only broke off from a holiday at his Texas ranch yesterday - and that the war in Iraq had left America unable to tackle emergencies at home.

The Pentagon said it would send an extra 10,000 National Guardsmen into Mississippi and Louisiana, bringing the force to more than 18,000, and had ordered an amphibious assault ship back from the Gulf to help with the relief operation.

"We've got the resources necessary to do two separate things," Mr Bush told ABC. "I hope people don't play politics during this time. This is a natural disaster the likes of which this country may not have seen before. What we need to do is to come together as a nation... there will be ample time for politics."

Ray Nagin, the Mayor of New Orleans, said last night that thousands had probably died in the floods brought by the storm - although that appears to be largely a guess since bodies are still not being routinely collected or counted.

Mr Nagin ordered virtually the entire police force to abandon search-and-rescue efforts and stop thieves who were becoming increasingly hostile. "They are starting to get closer to heavily populated areas - hotels, hospitals, and we’re going to stop it right now," he said.

Tempers also were starting to flare along the rest of the Gulf Coast strip. Police said a man in Hattiesburg, Mississippi, shot and killed his sister in a row over a bag of ice. Dozens of carjackings were reported, including a nursing home bus and a truck carrying medical supplies for a hospital.

Chris Ayres, a Times reporter, travelled from the devastated Mississippi town of Biloxi to Mobile, Alabama, and said: "There is a feeling of total lawlessness which is quite frightening. There is a feeling that there are no property rights, no rights at all. Along the road people have broken into abandoned hotels and are just living there.

"But the scenes at the petrol stations are the most unsettling. People are getting really annoyed and some petrol stations, especially those closest to the coast, have posted armed guards. Things are getting out of hand."

Authorities are trying to evacuate all civilians from New Orleans, where an estimated 80,000 people ignored an order to leave last week ahead of Katrina's arrival.

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