Aid operations have been stepped up and are starting to ease the plight of victims in the hurricane-devastated city of New Orleans.
Military convoys have been arriving with supplies of food, medicine and water, and evacuations are continuing.
Tens of thousands of people, mainly from the poor African-American areas of the city, are still stranded, five days after the storm struck the Gulf Coast.
President George W Bush is due to address the nation on the crisis.
The decision to deliver the address live from the White House is being seen as a reflection of the pressure that he is under.
The Senate is opening an investigation into how the disaster has been handled.
On Friday, Mr Bush visited the affected area.
He toured parts of Alabama and Mississippi, before taking a helicopter flight over the Louisiana city.
Mr Bush admitted that the initial response had not been acceptable, but he said progress was now being made.
But correspondents say the arrival of relief convoys has not defused the anger in New Orleans.
The BBC's Adam Brookes says that if casualty figures start to mount, President Bush will come under more pressure to explain what many see as a huge failure of America's emergency response system.
Cheers and tears
National Guard convoys managed to deliver long-awaited supplies to the New Orleans Convention Centre on Friday.
They were met with cheers, tears and frustration from some of the 20,000 people huddled amid the filth and the dead.
"They should have been here days ago," Michael Levy said.
Troops stockpiled supplies and distributed bottled water and ration packs, as a hospital was evacuated.
The city mayor said he was "cautiously optimistic" after meeting Mr Bush. "I feel like we've gotten everyone's attention," Ray Nagin said.
The evacuation of New Orleans is expected to take several days.
National Guards halted the emptying of the Superdome stadium on Saturday after buses stopped coming, but the 2,000 people left remained calm.
Most flights out of the stricken area will take refugees to Texas, which is providing emergency shelter for 75,000 survivors in Houston, Dallas and San Antonio.
Relief supplies have also started to arrive in the neighbouring state of Mississippi, where tens of thousands have been made homeless by the storm.
Mr Bush has signed a $10.5bn (£5.7bn) emergency aid package passed by Congress.
More than 44 foreign governments and international organisations have offered help. Cuba and Venezuela put aside their differences with the Bush administration to offer assistance.
Meanwhile, Treasury Secretary John Snow has said that the financial impact of Hurricane Katrina may not be as bad as first feared and will not cause significant change in long-term economic prospects.
He described the impact of the storm as mind-boggling, but said that both he and the chairman of the Federal Reserve, Alan Greenspan, felt the inherent strength of the economy meant that growth would only be slowed for three to four months.
Five days after the hurricane struck, the scale of the casualties is still not known.
However one Louisiana senator has predicted the death toll could climb above 10,000 in the state alone.
Army engineers have begun work on New Orleans' breached levees, but say it will take up to 80 days to pump the flood waters from the low-lying city.
Looting has swept the city. There have also been outbreaks of shootings and car hijackings, and reports of rapes.
In Washington, senators said they would launch an inquiry on Wednesday into the federal response to Katrina and emergency preparedness.
Some African-American leaders in Congress have also criticised the administration's approach.
"We cannot allow it to be said by history that the difference between those who lived and... died... was nothing more than poverty, age or skin colour," congressman Elijah Cummings said.