The Mayor of New Orleans has ordered most of the city's police to abandon rescue efforts to focus on quelling the widespread looting and violence.
Looters have targeted homes, shops and offices
There are reports of shootings, carjackings and thefts across the city smashed by Hurricane Katrina.
Louisiana's governor asked Washington to send more people to help with the relief mission, to free National Guard troops to concentrate on looters.
President Bush condemned the acts and called for "zero tolerance".
Dozens of law officers were being brought in from around the country, the governor's office said.
Governor Kathleen Blanco said she was "furious" about the growing crime wave.
"We will do what it takes to bring law and order to our area."
Fires and foreboding
The BBC's Alastair Leithead in New Orleans says there is a feeling of foreboding as those marooned become more desperate.
Police have faced shoot-outs and some reports say that martial law has been imposed across parts of the city.
Armed gangs have moved into some hotels.
People have been breaking into shops, houses, hospitals and offices buildings, as gunshots rang out and fires flared.
Thieves used a forklift truck to break into one city pharmacy, the Associated Press reports.
Dozens of carjackings have been reported, including a nursing home bus and a truck carrying medical supplies for a hospital.
"We're going to stop it right now," Mayor Ray Nagin said.