Thousands of people were feared drowned in the flooded US city of New Orleans after Hurricane Katrina wrecked havoc, triggering a mass exodus by survivors.
There has also been rampant looting.
With flood waters rising steadily, submerging up to 80 per cent of the ruined city, authorities ordered a 'complete evacuation' and mounted a massive relief operation to tackle 'one of the worst natural disasters' the country has seen.
President George W Bush undertook an aerial survey of the disaster zone and said it would take 'years' for the country to recover and repair the damage.
"We're dealing with one of the worst natural disasters in our nation's history. This recovery will take a long time. This recovery will take years," Bush said.
He promised vast assistance to face the grim situation and said the relief effort would initially focus on restoring power, communication and transport links.
Rescue workers pushed harder to locate survivors in the city, which looked increasingly like a battle zone.
Though the exact number of the dead was not available, New Orleans Mayor Ray Nagin said thousands have perished in the disaster.
"We know there is a significant number of dead bodies in the water," as well as other people dead in attics." Asked how many dead, he said, "Minimum, hundreds. Most likely, thousands."
Looters went on a rampage in the ghost-city, forcing authorities to pull 1,500 policemen out of rescue and rescue operations to maintain law and order.
Residents reported hundreds of looters on the streets, armed robberies and carjacking. Several shots were also fired across the city, they said.
With food and water running short and tempers running high, the mayor ordered "complete evacuation", warning that the city would be out of commission for two to three months and residents not allowed back for one or two months.
Authorities also started moving people taken shelter in the Superdome to Houston, about 550 kilometres away, as conditions became "desperate" with food and other supplies running out, toilets overflowing, air becoming foul and temperature touching 37 degrees Celsius.
The wounded were being evacuated by military transport planes.
Officials engaged in relief work said it could take weeks or even months to drain out the water, which gushed in when huge waves breached the levees at three places.
Authorities are also worried about an outbreak of water-borne epidemics like cholera and typhoid and other dehydrating diseases.
In Mississippi, which took a direct hit from Katrina, at least 110 people have been confirmed dead, while officials fear the toll would rise.
More than two million people in three badly affected states of Louisiana, Mississippi and Alabama were still without electricity and the utilities' crews were working round the clock to restore power.