Engineers have begun pumping water from New Orleans after repairing a key breach in the city's floodwalls, eight days after Hurricane Katrina struck.
Some New Orleans people living on higher ground have stayed put
Emergency teams are trying to persuade remaining residents to leave, warning the city is no longer habitable.
Some evacuees are to be moved on to specially chartered cruise ships, which could be their home for six months.
President George W Bush has invited top US politicians from both parties to the White House to discuss the situation.
The response to Hurricane Katrina is expected to head the agenda for the US Senate when it reconvenes on Tuesday after its month-long summer break.
Two Senate committees are shortly to begin hearings into how the relief effort was handled, with Democratic Senator Hillary Clinton leading calls for a wider-ranging commission of inquiry.
In other developments:
- Environmental experts warn that human sewage and chemical pollution from the flooded city could create a second disaster if they are pumped out untreated into Lake Pontchartrain and the Mississippi
- Two United Nations aid agencies, Unicef and the World Health Organisation, send teams of specialists to Texas and Georgia to help the US federal emergency effort
- The White House says it will not "engage in the blame game" after New Orleans newspaper Times-Picayune demands the sacking of top emergency service officials in an open letter.
In New Orleans, the huge operation to repair its damaged floodwalls and levees is starting to show signs of progress, officials say.
Pumps brought in specially have begun working at the 17th Street Canal since the breach there was sealed using rock and sandbags, US army engineers spokesman George Stringham told the BBC News website on Tuesday.
The pumps are moving water out of the canal and back into Lake Pontchartrain.
The 17th Street breach was the cause of much of the flooding but hard work remains to be done to repair damage to other floodwalls built to keep out the lake's waters.
Workers are also looking for bodies, some of which are being put in refrigerated lorries and portable morgues before the identification process begins.
One New Orleans police officer told the BBC bodies were being tied up on poles to stop them floating away before the coroners could reach them.
New Orleans Mayor Ray Nagin told NBC's Today Show that a figure of 10,000 deaths "wouldn't be unreasonable".
Officers have called for the several thousand people thought still to be in the city to leave while the clean-up goes on.
Lt Lawrence McLeary of Louisiana State police said one of the biggest challenges was persuading people remaining in the city to leave their homes.
The lack of provisions and power and unsanitary conditions, he added, made the city "just not safe to stay" in.
Texan officials are to move some 7,000 evacuees out of Houston and on to cruise ships chartered by the government for up to six months.
Priority will be given to the elderly and those with special health problems.