A public health emergency was declared for the entire Gulf Coast Wednesday as New Orleans officials estimated that hundreds, if not thousands, of city residents were killed by Hurricane Katrina (search) and the storm's aftermath.

"We know there is a significant number of dead bodies in the water," and other people dead in attics, New Orleans Mayor Ray Nagin said. Asked how many, he said: "Minimum, hundreds. Most likely, thousands."

If that's true, it would make Katrina the deadliest hurricane to hit the United States in more than a century.

President Bush called the storm "one of the worst natural disasters in our nation's history" when he addressed the nation about Katrina's destruction Wednesday evening from the White House Rose Garden.

"This recovery will take a long time," he said. "This recovery will take years."

Calling life in the aftermath of Katrina "very dangerous," federal officials said they rushed food, medicine and water to the victims as part of a wide-ranging government rescue-and-relief response.

As the flooding in New Orleans grew worse by the minute Wednesday, engineers scrambled to plug two broken levees and rescuers searched for survivors along the Gulf Coast.

Federal Emergency Management Agency Director Mike Brown warned that structural damage to homes, diseases from animal carcasses and chemicals in floodwaters made it unsafe for residents to come home anytime soon.

"We're going to need your patience," said Brown Wednesday evening. "We're going to need your endurance."

Before Bush spoke about the government's recovery efforts, he flew over the devastated areas in Air Force One on his way back to the White House from his ranch in Crawford, Texas. Bush may visit areas hardest hit by Katrina on Friday or Saturday.

"It's totally wiped out," Bush told aides at one point during the flight.

Louisiana Gov. Kathleen Blanco (search) has ordered everyone in New Orleans — many of whom have been huddled in the Superdome and other rescue centers — to leave. As many as 25,000 people were going to be bused from the Superdome to the Houston Astrodome in Texas. Click here for more on that story.

There will be a "total evacuation of the city. We have to. The city will not be functional for two or three months," Nagin said.

Nagin estimated 50,000 to 100,000 people remained in New Orleans, a city of nearly half a million people. He said 14,000 to 15,000 a day could be evacuated.

But it was still unclear where the people would go.

"There's a whole bunch of misery and everything else" in New Orleans, one refugee told FOX News. "How the hell can they leave when they don't have a way to leave? They need rescue now — not tomorrow, now."

The air conditioning inside the Superdome was out, the toilets were broken, and tempers were rising in the sweltering heat. Sewage could be seen seeping out of manholes in downtown New Orleans.

"We have a huge problem and until we have that breach under control, we will continue to have a problem," Blanco told FOX News Wednesday morning.

"One of the reasons we're asking people to leave is it's getting more and more difficult to get things like water and basic things to them … it's just a very trying time for everybody. We're trying to minimize the impact. We've got to vacate the people out of the Superdome today and tomorrow — that's about 20,000 people that need to be moved."

Sections of Interstate 10, the only major freeway leading into New Orleans from the east, lay shattered, dozens of huge slabs of concrete floating in the floodwaters.

In some counties, search and rescue teams were marking an "X" on homes and roofs where dead bodies were found so that they can later be retrieved.

"There are casualties in New Orleans … Numbers are not a priority at this time because we are focusing our efforts on saving the lives that are still there. There are people trapped in their attics and the water is still rising," said Lt. Kevin Cowan, spokesman for the Louisiana Office of Emergency Preparedness. "We want everyone to get out. It is not safe in New Orleans."

Energy Secretary Sam Bodman has approved the release of oil from the federal Strategic Petroleum Reserve (search) to help refiners affected by Katrina. The announcement helped push oil prices lower.

Click here for more on that story.

Health and Human Services Secretary Michael Leavitt said during a press conference Wednesday that his agency is concerned about potential disease outbreaks and was sending medical experts from the Centers for Disease Control and Prevention. He urged residents of the coastal area to boil water and follow food safety precautions as well as to avoid situations that might lead to carbon monoxide poisoning from electricity generators.

He also said that mental health personnel were being sent to the area.

"The situation in all affected areas remains very dangerous," said Homeland Security Secretary Michael Chertoff. "We will work tirelessly to ensure that our fellow citizens have the sustained support and the necessary aid to recover and reclaim their homes, their lives and their communities."

Officials said the death toll from Hurricane Katrina had reached at least 110 in Mississippi, and officials are "very, very worried that this is going to go a lot higher," said Joe Spraggins, civil defense director for Harrison County, home to Biloxi and Gulfport. In neighboring Jackson County, officials said at least 10 deaths were blamed on the storm.

The Pentagon is mobilizing possibly an unprecedented rescue-and-relief mission, sending Navy ships, Army helicopters and the USNS Comfort hospital ship from Baltimore. But the ships could take days to get to the Gulf Coast.

Countries such as Great Britain and Germany have also offered assistance.

"Our thoughts are with the people in the states affected by Katrina and our condolences to the relatives of those who have lost their lives," a spokesman for British Prime Minister Tony Blair told FOX News. "We have not received any requests for assistance but we stand ready to help if asked."

Katrina, which was downgraded to a tropical depression, packed winds around 30 mph as it moved through the Ohio Valley early Wednesday, with the potential to dump 8 inches of rain and spin off deadly tornadoes.

The remnants of Katrina spawned bands of storms and tornadoes across Georgia that caused at least two deaths, multiple injuries and leveled dozens of buildings. A tornado damaged 13 homes near Marshall, Va.

Water Rises in the Big Easy

As the waters continued to rise in New Orleans, four Navy ships raced toward the Gulf Coast with drinking water and other emergency supplies, and Red Cross workers from across the country converged on the devastated region. The Red Cross reported it had about 45,000 people in 200 shelters across the area.

The U.S. Coast Guard has rescued over 1,200 people from rooftops as rescue workers toil around the clock with the help of Navy, U.S. Customs, Marine and Army helicopters.

"There are hundreds still out there on rooftops," Coast Guard Capt. Bruce Jones told FOX News on Wednesday. "Every one of our swimmers is scratched and bruised and cut and each and every one of them is ready and raring to go out again. It's everything I can do to get them to get a few hours of sleep."

To repair damage to one of the levees holding back Lake Pontchartrain, the Army Corps of Engineers said it planned to use heavy-duty Chinook helicopters to drop 3,000-pound sandbags Wednesday into the 500-foot gap in the failed floodwall. But the agency was having trouble getting the sandbags and dozens of 15-foot highway barriers to the site because the city's waterways were blocked by loose barges, boats and large debris.

Officials said they were also looking at a more audacious plan: finding a barge to plug the 500-foot hole.

It could take close to a month to get the water out of the city. If the water rises a few feet higher, it could also wipe out the water system for the whole city, said New Orleans' homeland security chief, Terry Ebbert.

"The challenge is an engineering nightmare," Paul Accardo, a police spokesman.

"We are looking at 12 to 16 weeks before people can come in," Nagin said on ABC's "Good Morning America, "and the other issue that's concerning me is have dead bodies in the water. At some point in time the dead bodies are going to start to create a serious disease issue."

Louisiana Rep. Rodney Alexander, whose congressional district lies to the north of New Orleans, told FOX News that many people are coming to his area, which is on higher ground.

"We are dealing mostly with evacuees that are now becoming refugees. Reality has sunk in that this is going to be something we're going to be dealing with for months to come," Alexander told FOX News.

His district is preparing gymnasiums, auditoriums, churches and other buildings to serve as temporary shelters.

"We're trying to provide as much space as necessary to provide food and medical attention," Alexander said.

On New Orleans' (search) Canal Street, dozens of looters ripped open the steel gates on clothing and jewelry stores and grabbed merchandise. The looting prompted authorities to send more than 70 additional officers and an armed personnel carrier into the city. One police officer was shot in the head by a looter but was expected to recover, authorities said. Blanco said she will ask President Bush for military troops to help keep looting under control.

A giant new Wal-Mart in New Orleans was looted, and the entire gun collection was taken, The Times-Picayune reported. "There are gangs of armed men in the city moving around the city," said Ebbert, the city's homeland security chief. Also, looters tried to break into Children's Hospital, the governor's office said.

'It Was Complete Chaos'

In Biloxi, Miss., people picked through casino slot machines for coins and ransacked other businesses. In some cases, the looting was in full view of police and National Guardsmen.

Several victims in Harrison County were from a beachfront apartment building that collapsed under a 25-foot wall of water as Katrina slammed the Gulf Coast with 145-mph winds Monday. Louisiana officials said many were feared dead there, too, making Katrina one of the most punishing storms to hit the United States in decades.

Blanco asked residents and America to spend Wednesday in prayer.

"We'll save the city. We will rebuild. We will restore. We are very sturdy people who have met adversaries in the past and we will meet it today. It's going to take some time, it's going to take some effort, it's going to take a lot of help from the nation," she told FOX News. "We have our work cut out for us in the coming days and weeks."

She estimated that about 1 million Louisiana residents have been left without homes.

According to New Orleans power grid managers, it will take six to eight weeks to get power back on in the city after the water is drained. Water service likely won't be reestablished until a week or so after that, at the earliest.

Louisiana Lt. Gov. Mitch Landrieu said 3,000 people have been rescued by boat and air, some placed shivering and wet into helicopter baskets. They were brought by the truckload into shelters, some in wheelchairs and some carrying babies, with stories of survival and of those who didn't make it.

"Oh my God, it was hell," said Kioka Williams, who had to hack through the ceiling of the beauty shop where she worked as floodwaters rose in New Orleans' low-lying Ninth Ward. "We were screaming, hollering, flashing lights. It was complete chaos."

A helicopter view of the devastation over Louisiana and Mississippi revealed people standing on black rooftops, baking in the sunshine while waiting for rescue boats.

"I can only imagine that this is what Hiroshima looked like 60 years ago," said Mississippi Gov. Haley Barbour (search) after touring the destruction by air Tuesday.

Across Louisiana, Mississippi and Alabama, more than 1 million residents remained without electricity, some without clean drinking water. An untold number who heeded evacuation orders were displaced and 40,000 were in Red Cross shelters, with officials saying it could be weeks, if not months, before most will be able to return.

"We need to save lives right now, there may be thousands still stranded out there. Folks need to stay exactly where they are … and ultimately, FEMA and the National Guard and the other agencies will find them," former FEMA Director Joe Allbaugh told FOX News. "It's devastating, that's the only way you can describe it."

The Red Cross said it so far has raised $21 million, a figure comparable to the response for tsunami victims following the devastation in Asia earlier this year. Nearly $15 million of that has come from individual donations through the Red Cross Web site, with the rest representing corporate contributions.

FEMA (search) was considering putting people on cruise ships, in tent cities, mobile home parks, and so-called floating dormitories — boats the agency uses to house its own employees.

The relief effort going on now is "one of the most massive coordinations, perhaps in the history of the federal government" working with state and local officials, FEMA Deputy Director Patrick Rhode told FOX News Wednesday morning, adding that communication was improving every hour.

"It's true that we are considering all [housing] options at this point. When you look at the scale of the damage of what we're looking at ... I think it would do us all well to consider all options right now."

It's currently impossible to even begin to estimate the death toll, however, he said.

"This is unfortunately going to be a several-day effort until we truly know what the situation is," Rhode said.

Alabama Lt. Gov. Lucy Baxley said her states was "blessed" with having far less devastation and life-loss than that of neighboring states.

"This is beyond human capacity and human comprehension and we just really need to rely on our faith," she told FOX News. "This is just a horrible thing and in Alabama, we're so much better off than our neighboring states ... we have nothing in comparison to what Mississippi and Louisiana have ... my state will do everything it can to ease the suffering, not only for people in Alabama" but for its neighbors, as well.

FOXNews' Jeff Goldblatt, Adam Housley, Phil Keating, Rick Leventhal and Liza Porteus and The Associated Press contributed to this report.