US Federal emergency teams have begun reaching isolated areas in the south of the country that have been devastated by Hurricane Katrina.
Hundreds of people are feared dead in Mississippi, and the Louisiana city of New Orleans is badly flooded.
Flooding in the city, most of which is already under water following the storm, is expected to worsen in some areas after an effort to plug a breach in a levee last night failed.
Officials are planning to evacuate the Superdome stadium because conditions have become unfit for the 20,000 people who have taken shelter there.
The stadium is without power, and there are difficulties delivering supplies of food and water to those stranded.
It is reported heavily armed police have been trying to impose a form of martial law to stem outbreaks of looting in the city.
The mayor of New Orleans has warned it could be up to four months before evacuated residents could return to the city.
Emergency oil reserve
Meanwhile, the US government has said it will release oil from an emergency reserve in a bid to ease international worries of shortages caused by Hurricane Katrina.
US President, George W Bush, has cut short his holiday to return to Washington to help oversee the recovery effort.
Fears of a major death toll have grown after the scope of the devastation became more apparent.
The US military has added Navy ships, two helicopter assault vessels and a hospital ship as well as dozens of helicopters to a massive relief effort in the wake of the disaster.
A million people were without power across Mississippi, Louisiana and Alabama, water supplies were compromised and collapsed communications left large areas cut off from the outside world.
Officials warned of large numbers of gas leaks. Fires could be seen burning out of control in several areas.
Thousands of people travelling south to return to their homes have been turned back by police who closed off access to all but emergency vehicles and imposed a curfew.
