New Orleans Mayor Ray Nagin has issued a "desperate SOS" for thousands of people stranded with no food or water at the city's convention centre.
Up to 25,000 people are at the centre, in addition to tens of thousands more still trapped by the flood waters unleashed by Hurricane Katrina.
The state governor has called for 40,000 troops to restore order after a spate of lawlessness, AFP reports.
President George W Bush plans to visit the Gulf coast disaster zone on Friday.
Launching an appeal for financial help, President Bush said this was an agonising time for the people there but he promised help was on its way.
Hundreds or even thousands of people are feared to have drowned in New Orleans, most of which is under water. In Mississippi, 110 people are confirmed dead, but officials warn the toll is expected to rise.
According to the White House, nearly 155,350 sq miles (250,000 sq km) has been affected by the hurricane - an area roughly the size of the UK.
In New Orleans, people made homeless by the flooding have grown increasingly desperate, with outbreaks of shootings, carjackings and thefts.
Police chief Eddie Compass said there were reports of women being raped.
He told AP he sent 88 officers to quell the unrest at the convention centre but they were beaten back by an angry mob.
State governor Kathleen Blanco has said she is "furious" at the unrest and vowed to restore order.
Squalid conditions
Earlier, medical evacuations from the city's Superdome stadium were disrupted after reports that a gun shot was fired at a rescue helicopter. Similar reports have come from the city's hospitals.
"We are out here like pure animals. We don't have help," Rev Issac Clark told the Associated Press news agency outside the convention centre, where dead bodies are still lying in the open.
People were chanting: "We want help".
"Right now we are out of resources at the convention centre and don't anticipate enough buses," Mayor Nagin said in a statement read out by CNN.
"Currently the convention centre is unsanitary and unsafe and we are running out of supplies for 15,000 to 25,000 people."
There is no electricity in the city, and people who have lost everything are struggling to find food and clean water, reports the BBC's Alastair Leithead in New Orleans.
Homeland Security Secretary Michael Chertoff said more national guards were being sent to New Orleans in the next few days - more than quadrupling the 2,800 already there.
The mayor has ordered a total evacuation and warned it will be months before people can return to their homes.
More than a million people fled before the hurricane arrived on Monday, but at least 100,000 were unwilling or unable to leave.
At the city's Superdome stadium, the numbers of people seeking refuge has swelled to at least 20,000 and conditions there are appalling. Mass evacuations are under way.
The first bus-loads of people have arrived at Houston's Astrodome stadium in Texas, 560km (350 miles) away, where beds and blankets for up to 25,000 people have been set up.
The most vulnerable are going to the Louisiana state capital, Baton Rouge.
In Mississippi, curfews are in place in the hard-hit towns of Biloxi and Gulfport as the authorities try to prevent the scale of looting seen in New Orleans.
