US police in the hurricane-stricken city of New Orleans have shot eight people, reportedly killing five, as a massive rescue effort continues.
The incident occurred when contractors escorted by officers were fired at.
The authorities have vowed to restore security in the city following a breakdown of law and order.
Rescue teams are working around the clock to move stranded survivors to safety - although some residents appear determined to stay behind.
Sunday's shooting took place while 14 contractors were crossing the Danziger Bridge under police escort, New Orleans Deputy Police Chief WJ Riley said.
He said the contractors came under fire from gunmen and police officers shot back, killing at least five of them.
None of the contractors were killed.
In a separate incident, a helicopter with two civilians on board crashed in the city.
The two people on board, who are said not to have been involved in the rescue operation, got away with minor injuries.
Homeland Security Secretary Michael Chertoff said troops had secured the city and full relief operations were underway.
Helicopters and boats are still looking for survivors in flooded neighbourhoods.
Even though only 59 bodies have been recovered so far in New Orleans, it is believed the death toll could run into the thousands.
"I can't tell you what the numbers are going to be, but I think we need to prepare the country for what's coming," Mr Chertoff said on Sunday.
"We are going to uncover people who died, maybe hiding in houses, got caught by the flood... It is going to be about as ugly of a scene as I think you can imagine."
The authorities has issued a mandatory evacuation order.
The Coast Guard made a media appeal for all those stranded to hang out bright or white clothes to draw attention.
But the BBC's Richard Greene says he met several people who were determined to stay behind.
Mr Chertoff has warned that staying is not a reasonable alternative.
"We are not going to be able to have people sitting in houses ... for weeks and months while we de-water and clean the city," he said.
"The flooded places, when they are de-watered, are not going to be sanitary."
Earlier, senior members of the administration visited the devastated Gulf Coast, amid accusations that the federal response to the crisis had been inadequate.
Defence Secretary Donald Rumsfeld - who was in New Orleans - said it would take "many months, and into years" for the city to recover.
Secretary of State Condoleezza Rice, for her part, toured Mobile, Alabama.
She said it had been the sheer scale of the disaster that had made the government's response seem slow.
Meanwhile, the US has given the EU and Nato a list of specific emergency aid it needs for the relief operation.
American officials have asked for blankets, first aid kits, water trucks and food.
EU Environment Commissioner Stavros Dimas said the union was ready to offer whatever assistance it could, while Nato said it too was ready to help.
More than one million people are said to have had to leave their homes in Louisiana, Mississippi and Alabama. Most of them are in Texas, Tennessee, Indiana and Arkansas.