Long traffic queues have formed on the approaches to suburban New Orleans as residents who fled Hurricane Katrina's floods return to inspect their homes.
Engineers finally sealed one of the levees breached by the storm as rescuers sought survivors in the city.
New Orleans Mayor Ray Nagin told NBC's Today Show that a figure of 10,000 deaths "wouldn't be unreasonable".
President George W Bush has been in the region again and two ex-presidents have launched a national disaster appeal.
Mr Bush has also declared a state of emergency in 10 US states, thus enabling federal funds to be made available there to assist with the evacuees.
New Orleans Deputy Police Warren Riley warned people remaining in the city on Monday to leave:
"Our officers are basically telling people there is absolutely no reason to stay here... We advise people that this city has been destroyed."
As searchers look for bodies in the ruined city, refrigerated lorries and portable morgues are on hand.
George Bush Senior, father of the current president, and Bill Clinton formally launched the Bush-Clinton Katrina Fund for disaster relief at a meeting in Houston, Texas.
Mr Clinton and his wife, New York Senator Hillary Clinton, have raised the idea of a 9/11 style Congressional inquiry into Hurricane Katrina and the federal government's response.
The idea appeared more a passing thought than a formal request but will increase political pressure on the Bush administration, the BBC's Michael Buchanan notes.
Meeting Katrina victims in Baton Rouge, Louisiana, President Bush thanked families who had taken in the homeless and praised the work of churches, synagogues and mosques.
His suggestion over the weekend that local officials had made mistakes led one Louisiana Senator, Mary Landrieu, to threaten to punch him if he said it again.
'Punched in the gut'
Army engineers and contractors have now sealed one of the three levee gaps left by Katrina, at the 17th Street Canal, US Army Corps of Engineers spokesman John Hall confirmed for the BBC News website.
Work is still under way to close breaches at two other canals - London Avenue and the Inner Harbor Navigation Canal.
Long lines of vehicles formed outside Jefferson Parish, a suburban area of 500,000 on the edge of New Orleans, on Monday after residents were allowed to return briefly to their homes.
Roads opened at 0600 local time (1100 GMT) and are due to remain open for 12 hours, officials said.
Seeing his home caked in mud and mould, resident Gabriel Whitfield, 36, said it was like being "punched in the gut".
"It's hard to see something you worked so hard to accomplish just wiped out," he said.
Homeland Security Secretary Michael Chertoff has warned that flooded places will remain unsanitary even after being drained.
Fewer than 10,000 people remain in the city, according to an army estimate based on aerial reconnaissance.
Thousands of regular troops and National Guards are on the streets to quell further outbreaks of looting and violence in the Louisiana city.
More than one million people are said to have left their homes in Louisiana, Mississippi and Alabama. Most of them have gone to Texas, Tennessee, Indiana and Arkansas.