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.
There would be a total evacuation of the city, he said, warning it could be months before residents could return.
President George W Bush said the area could take years to recover.
Cutting short a holiday in Texas to take charge of the federal recovery effort, Mr Bush said the government was dealing with one of the worst natural disasters in US history.
"This is going to be a difficult road, the challenges we face on the ground are unprecedented, but there's no doubt in my mind that we'll succeed," he said.
Mr Bush, whose Air Force One plane flew low over the affected area, was taken aback by the scale of the disaster.
"I can't tell you how devastating the sites were," he said.
With conditions still deteriorating, the government has declared a public health emergency along the whole of the Gulf coast, to speed up the delivery of food, water and fuel to the region.
Homeland Security Secretary Michael Chertoff said 1,700 truckloads of essential supplies were on their way there.
Medical shelters are being set up offering 10,000 beds, while the US military is providing dozens of rescue helicopters and boats.
The Pentagon has ordered 10,000 extra national guardsmen to Louisiana and Mississippi.
Mayor Nagin said he had no choice but to remove the 50,000 to 100,000 people left in the city when the hurricane struck.
"We have to. The city will not be functional for two to three months," he said.
He said dead bodies in the water would soon create a serious disease problem.
Survivors could be taken out at a rate of up to 15,000 a day.
Counting the dead remains a secondary priority until they are rescued, officials say.
In Mississippi, officials have warned the death toll is likely to climb above the current 110.
Harrison County bore the brunt of Hurricane Katrina as it slammed into Biloxi and Gulfport before heading inland.
New Orleans has been plagued by looting, floods and increasingly desperate people, two days after the hurricane.
About a million people evacuated from the area before the hurricane struck, but tens of thousands of others are trapped in the city.
The authorities are planning to evacuate up to 20,000 people from the city's Superdome stadium where sanitary conditions are said to be appalling.
Four people believed to be elderly or infirm died there overnight, according to reports.
Surges of flood water submerged more areas of the city after failed attempts to plug breaches in the barriers which are supposed to protect it.
The pumps which usually assist are no longer working because of the rising levels.
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.
While some looters are stealing non-essential goods, others are simply trying to find food and water.
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.