Hurricane Katrina is thought to have killed hundreds, probably thousands of people in New Orleans, the city's mayor, Ray Nagin, has said.
Mr Nagin said there were significant numbers of corpses in the waters of the flood-stricken city, while many more people may be dead in their homes.
The city has been plagued by looting, floods and increasingly desperate people, two days after the hurricane.
State officials say they will rescue survivors before counting the dead.
Mr Nagin warned it could be four months before people could return home.
In Mississippi, the death toll has reached 110, but there are still no official estimates for Louisiana.
Map of central New Orleans
The head of the US Department of Homeland Security, Michael Chertoff, said life remains "very dangerous" in the aftermath of the hurricane.
A public health emergency has been declared to speed up the delivery of food, water and fuel to the region. Medical shelters are being set up offering 10,000 beds, while the US military is providing dozens of rescue helicopters and boats.
The authorities are planning to evacuate up to 20,000 people from the city's Superdome stadium where conditions have sharply deteriorated. Water is rising inside, there is no electricity and toilets are overflowing.
Four people believed to be elderly or infirm died in the stadium overnight, according to reports.
In Mississippi, officials have warned the death toll is likely to climb further.
Harrison County bore the brunt of Hurricane Katrina as it slammed into Biloxi and Gulfport before heading inland.
"I can only imagine that this is what Hiroshima looked like 60 years ago," said the state's governor, Haley Barbour, after viewing the destruction from the air.
In the worst hit areas of Mississippi, 90% of the buildings were "totally just gone", he said.
About a million people were evacuated from the New Orleans area before the hurricane struck, but Mr Nagin said a "couple hundred thousand" were still trapped in the city.
Water from Lake Pontchartrain is pouring into New Orleans after the failed attempts to plug breaches in the barriers which are supposed to protect the city.
Follow Katrina's path
The pumps which usually assist are no longer working because of the rising levels.
"The water will rise to try and equal the water level of the lake, which is 3ft (1m) above sea level," Mayor Nagin said.
"I'm on the 27th floor of a very un-air conditioned building looking out over the city. And I'm looking uptown and where there was dry land, there is now several feet of water."
Earlier he voiced his frustration with the relief effort, attacking what he saw as a lack of co-ordination.
"There is way too many frigging ... cooks in the kitchen," he said.
The BBC's Alastair Leithead in New Orleans says there is panic as vital supplies run out. Heavily armed police have been trying to impose a form of martial law to stem looting.
The search for survivors continues in New Orleans as waters continue to rise
While some looters are stealing non-essential goods, others are simply trying to find food and water.
Louisiana Governor Kathleen Blanco acknowledged there was a severe problem with looting, but said officials had to focus on finding those who were still alive.
"We don't like looters one bit, but first and foremost is search and rescue," she said.
Survivors are being found all the time.
In Mississippi, two children who lost their parents were taken to safety. In New Orleans, people are still being winched from roofs.
The US Red Cross has mobilised thousands of volunteers for its biggest-ever natural disaster effort and federal emergency teams are being dispatched.
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President George W Bush, whose ratings have fallen to an all-time low in the latest Washington Post-ABC News poll, has cut his holiday two days short.
He is expected to chair a meeting on Wednesday evening of a White House task force set up to co-ordinate the relief effort.
Queen Elizabeth II has sent a message to Mr Bush saying she is "shocked and saddened" at the devastation caused by Katrina, and expressing her sympathy for US citizens.
Damage estimates of more than $25bn suggest it could be the US insurance industry's most expensive natural disaster ever.
The price of crude oil on the international market hit a record $70.85 a barrel due to the vulnerability of oil and gas fields in the Gulf of Mexico. The US energy secretary has announced that oil will be released from government petroleum reserves. http://news.bbc.co.uk/1/hi/world/americas/4201480.stm