New Orleans Mayor Ray Nagin has issued a "desperate SOS" to help thousands of people stranded with no food or water.
Up to 25,000 people are at the city's convention centre, with thousands more trying to survive hurricane flooding.
Some 300 "battle-tested" National Guardsmen have arrived in New Orleans from Iraq with orders to restore civic order and using a shoot-to-kill policy.
President Bush, who is to visit the disaster zone, has requested $10.5bn (£5.7bn) emergency funds from Congress.
Congress is expected to approve the aid - described by the White House as a "stopgap measure" - quickly, in order to fund rescue and relief efforts in coming weeks.
'Locked and loaded'
Announcing the deployment of National Guardsmen in an effort to quell growing lawlessness within New Orleans, Louisiana Governor Kathleen Blanco said the men heading to the city were carrying deadly weapons and were ready to use them.
"They have M-16s and are locked and loaded. These troops know how to shoot and kill and I expect they will."
The deployment came as the thousands stranded in New Orleans endured another night of hot, dark fear.
People made homeless by the flooding have grown increasingly desperate, with outbreaks of shootings, carjackings and thefts.
Horrific reports have emerged from the Louisiana Superdome, where up to 20,000 people have been corralled, mainly without power and sanitation, since Hurricane Katrina struck.
"People were raped in there. People were killed in there. We had multiple riots," one police officer told the AFP news agency.
Desperate survivors awaited evacuation from the crippled city, while emergency supplies of food and water remained in short supply.
Corpses were said to be rotting in the streets or floating through the floodwaters.
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.
Earlier, President Bush used an appeal launch to speak of an agonising time for the people in New Orleans but he promised help was on its way.
Congressional House Speaker Dennis Hastert angered Gov Blanco by questioning the wisdom of spending billions of dollars rebuilding New Orleans below sea level.
"To kick us when we're down and destroy hope... is absolutely unthinkable," Ms Blanco replied.
According to the White House, about 90,000 sq miles (234,000 sq km) has been affected by the hurricane - an area roughly the size of the UK. Earlier, medical evacuations from New Orleans' 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.
"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."
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.
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.