Former US Secretary of State Colin Powell has said he fails to understand why better preparations were not made before Hurricane Katrina struck.
Katrina's dead are now coming to light across New Orleans
Two-thirds of Americans think President George W Bush could have done more to tackle the floods and damage in the south, one opinion poll suggests.
One member of Mr Bush's staff said such allegations "broke his heart".
With New Orleans still not completely evacuated, its police chief warned that to stay would be "suicidal".
Eddie Compass said officers would use the "minimum amount of force" necessary to persuade those who remain in the flooded city to leave.
"They were all insisting that I had to leave my home," one evacuee, Shelia Dalferes, told AP news agency.
"The implication was there with their plastic handcuffs on their belt. Who wants to go out like that?"
In other developments:
- Officials in New Orleans say after initial searches for bodies that the death toll there may be much smaller than the 10,000 predicted in some quarters
- Nato announces that its members will use their ships and aircraft to take aid to the disaster zones as a show of solidarity with the victims
- Ophelia, the weather front threatening Florida, is downgraded from a hurricane to a tropical storm after losing steam off the east coast
- The New Orleans Saints team are preparing to make San Antonio, Texas, their home ground for the beginning of the American football season, local media report.
President Bush is to begin a third visit to the disaster zone on Sunday, with stops in both Mississippi and Louisiana, a spokesman said.
American political figures in both the Republican and Democrat parties have accused the authorities of responding slowly.
There were "a lot of failures at a lot of levels - local, state and federal", Mr Powell told ABC.
"There was more than enough warning over time about the dangers to New Orleans - not enough was done," he said.
Mr Powell's views will be heard with particular interest as he is a highly respected figure and a prominent black American, the BBC's Justin Webb reports from Washington.
The Pew Research Center, which ran the new opinion poll, also indicated that two-thirds of the African-Americans questioned believed the government reaction would have been faster if most of those affected had been white.
But Mr Powell said so many African-Americans were left unprotected be cause they were poor, rather than because they were black.
It "should have been a blinding flash of the obvious... that when you order a mandatory evacuation, you can't expect everybody to evacuate on their own", he said.
Debate still rages in Washington into how independent the inquiries being launched into Katrina - led by President Bush himself and the Senate - will really be.
US Under-Secretary of State Karen Hughes stood by the president's response to the crisis.
"He's the president of all the people that we saw, who so desperately needed help and... wants to make sure that we get everyone the help they need, that we get New Orleans cleaned up and that we get about the business of rebuilding," she told the BBC.
Last to leave
As many as 10,000 of the original 450,000 residents were still believed to be in New Orleans by the time the mayor ordered the compulsory evacuation of the city.
The evacuation has been ordered for health reasons, with E.coli and other dangerous germs found in the polluted floodwaters.
Several deaths have already been attributed to infected wounds.
Floodwaters are dropping to reveal dead bodies, raw sewage, fuel and rusting vehicles, and more homes are becoming accessible to search and rescue teams.
On Thursday, Congress approved a bill providing $51.8bn (£28bn) for the relief effort, on top of $10bn granted last week.
The new funds will be used mainly to pay for search and rescue operations, drinking water and medical assistance, officials said.
Mr Bush warned Congress it should expect further emergency requests.
Speaking at the White House, Mr Bush announced that every family affected by Hurricane Katrina would receive the initial sum of $2,000 in aid.
He also declared Friday 16 September a national day of prayer and remembrance for the victims.