Rescue efforts continue along a large area of the Gulf of Mexico devastated by Hurricane Katrina. Louisiana's governor has ordered the complete evacuation of New Orleans and, given the rising death toll and preliminary assessments of damage, federal officials say this could be the worst disaster in U.S. history.

A boat sits on a house damaged by Hurricane Katrina

It may be weeks before authorities can even come close to stating the total death toll from Katrina. Rescue workers in the worst hit areas of Mississippi and Louisiana say they have seen dead bodies floating amid debris, but they generally do not try to retrieve them, concentrating instead on helping survivors, many of whom are stranded on roof tops or in the upper stories of buildings with no food, no clean water and no medical supplies.

After levees gave way in New Orleans Tuesday, 80 percent of the city was inundated. Louisiana Governor Kathleen Blanco called for the full evacuation of the city, much of which lies below sea level and relies on a system of levees, dikes and pumping stations to remain dry under normal conditions.

Louisiana has dealt with bad storms many times before, but Governor Blanco says this was something totally different. "What we are dealing with now is not what I would call normal hurricane damage," she said. "We are all very storm smart, I like to say, we have survived a lot, but this is catastrophic in ways we have not had to deal with."

Bryan Vernon and Dorothy Bell are rescued from their rooftop after Hurricane Katrina hit, causing flooding in their New Orleans neighborhood

The governor says the more than 500,000 residents of New Orleans who are now homeless may have to wait weeks just to assess their losses.

"I do not know if they will be able to move into their homes. If the engineering is done properly, they will be able to survey their homes," added Governor Blanco. "If we can dry the area out, if we can create passable streets, they may be able to survey their home, but probably it will be weeks before they can do that. Because of the devastation it is hard for me to say how many homes will be salvageable."

Federal officials are working closely with state agencies in both Louisiana and Mississippi to provide relief to the devastated area. Bill Lokey of the Federal Emergency Management Agency, known as FEMA, says the federal government is fully committed to the relief effort.

"It is going to take quite some time and the federal government will be here as long as necessary. I have my personal commitment from my job and my orders from the president and the director of FEMA," he said. "We are to do everything we possibly can, under our laws and regulations and under the authority of the federal government to bring whatever assistance we can."

President Bush has promised massive assistance to the victims of the hurricane. He is cutting short his vacation at his ranch here in Texas by two days in order to go back to Washington and oversee recovery operations.