And increasingly across the country, questions are being asked: "How could this happen?" "Why is help taking so long?" and "How can thousands of Americans be stranded?".

It is hot and humid in the city's stadium and toilets are overflowing

President George Bush was visiting some of the devastated areas of the south on Friday amid growing anger over the federal response to the disaster.

Officials insist their response has been effective - rejecting widespread criticism that the administration was too slow to react to the crisis.

There has also been criticism from opposition politicians and members of the public that spending on the war on Iraq diverted money away from flood-control projects.

New Orleans Mayor Ray Nagin has angrily accused Washington of not having a clue about what was going on.

On Thursday, he issued what he called a "desperate SOS" for help for up to 20,000 refugees stuck in a convention centre in New Orleans which he said was "unsanitary and unsafe" and running out of supplies.

I was in the tsunami region, and this response is incredibly more efficient, more effective and under the most difficult circumstances

Michael D Brown

Emergency response head

On Friday, authorities in Louisiana were trying to crank up the rescue operation. Convoys of school buses were trying to ferry out the thousands of people sheltering in the convention centre and the nearby New Orleans Superdome amid the filth and the dead.

The questions being asked focus on why it has taken so long to get those buses on the road - and why thousands of people sheltering in the places where they were told to take refuge are now going hungry and thirsty.

Louisiana Governor Kathleen Blanco says she has asked for a "Berlin drop" of food and water.

In an interview with Good Morning America on Friday, she said that they were finally starting to see the response from the federal authorities.

The task on the ground has been complicated by the frustration and despair felt by survivors who have gone days without essential supplies.

Much of the frustration has been directed at the national authority, the Federal Emergency Management Agency (Fema).

This is a national disgrace - we can send massive amounts of aid to tsunami victims, but we can't bail out the city of New Orleans

Terry Ebbert

"This is a national disgrace. Fema has been here three days, yet there is no command and control," Mr Ebbert said.

"We can send massive amounts of aid to tsunami victims, but we can't bail out the city of New Orleans."

One man, George Turner, who was still waiting to be evacuated, summed up much of the anger felt by the refugees.

"Why is it that the most powerful country on the face of the Earth takes so long to help so many sick and so many elderly people?" he asked.

Tens of thousands of people are still waiting to be evacuated

"Why? That's all I want to ask President Bush."

And John Rhinehart, the administrator of a New Orleans hospital without power and water, said: "I'm beginning to wonder if the government is more concerned about the looting than people who are dying in these hospitals."

There is widespread agreement among commentators that somewhere there has been a breakdown in the system.

The Biloxi Sun Herald in Mississippi asked: "Why hasn't every able-bodied member of the armed forces in south Mississippi been pressed into service?"

And on Friday the Washington Post wrote: "Though experts had long predicted that the city, which sits below sea level and is surrounded by water, would face unprecedented devastation after an immense hurricane, they said problems were worsened by a late evacuation order and insufficient emergency shelter for as many as 100,000 people."

Volunteer effort

The BBC's Jamie Coomarasamy in Baton Rouge says that on the ground in some areas, it is largely volunteers, including those from the Red Cross and other organisations, who are leading the relief efforts.

But he said these efforts were fairly haphazard, with local radio and television stations putting out requests for people to do what they could.

Fema head Michael D Brown has defended the federal response, saying that his agency had prepared for the storm, but that the widespread flooding had hampered the operation.

Could more have been done? I would say every society in the world is not preparing adequately for catastrophic events

Jan Egeland

UN emergency relief co-ordinator

"What the American people need to understand is that the full force of the federal government is bringing all of those supplies in, in an unprecedented effort that has not been seen even in the tsunami region," he said.

"I was in the tsunami region, and this response is incredibly more efficient, more effective and under the most difficult circumstances."

Jan Egeland, the UN emergency relief co-ordinator has written to US ambassador to the UN John Bolton offering help.

"I understand people's frustration, but I also know from bitter experience that this, the fifth and the sixth and the seventh days are always among the worst, because it is before you reach, really, the largest amount of people," he told the BBC.

"Could more have been done? I would say every society in the world is not preparing adequately for catastrophic events. Disaster prevention is something that we are campaigning for all over the world, and I would say no society is fully prepared for all eventualities."