Federal agencies stepped up relief efforts today in the aftermath of Hurricane Katrina, mobilizing to evacuate flood refugees from the Superdome in New Orleans, dispatching Navy ships and declaring a public health emergency for the entire Gulf Coast. The mayor of New Orleans said the death toll was at least in the hundreds and possibly in the thousands.

President Bush, addressing the nation from the White House Rose Garden after cutting short his month-long stay at his Texas ranch, warned, "This recovery will take a long time. This recovery will take years."

With floodwater levels in New Orleans apparently stabilizing, federal department heads led by Homeland Security Secretary Michael Chertoff earlier announced a series of measures to ramp up the relief effort. The steps included the dispatch of 50 helicopters, a U.S. Navy hospital ship and seven other naval vessels, swift-water rescue teams, a 500-bed mobile hospital and tons of military meals and other supplies.

In addition to National Guard troops already deployed, the Pentagon announced that 10,000 more Guard troops would be sent in the next two days to Louisiana and Mississippi, about 5,000 to each state.

The announcement came amid deteriorating security in New Orleans, where hundreds of looters roamed the streets with apparent impunity and gunfire could be heard in some neighborhoods. Many residents, desperate for food and drinking water, ventured out into murky floodwaters polluted by sewage, trash, fuel and debris from damaged buildings.

President Bush ended his vacation at his ranch near Crawford, Tex., two days early to fly back to Washington on Air Force One, passing over devastated areas on the way. "It's totally wiped out," he told aides at one point after viewing the destruction wrought by Katrina on Monday as it slammed ashore from the Gulf of Mexico with 140-mph winds and torrential rains.

The plane dropped as low as 2,500 feet over New Orleans, affording Bush a view of the Superdome surrounded by floodwaters, then flew over Mississippi, descending to 1,700 feet at one point. According to White House spokesman Scott McClellan, Bush said as he looked at the flooding in New Orleans, "It's devastating. It's got to be doubly devastating on the ground."

Surrounded by members of his Cabinet in the Rose Garden, Bush said, "We are dealing with one of the worst natural disasters in our nation's history." He gave a rundown of federal efforts, noting that the Federal Emergency Management Agency has deployed more than 50 disaster medical assistance teams and that the Coast Guard has rescued nearly 2,000 people to date, plucking many of them from rooftops with helicopters. He said more than 25 urban search and rescue teams totaling 1,000 personnel have been sent in.

At least 78,000 people are now in shelters, he said.

Bush announced formally that the Department of Energy will loan oil from the federal Strategic Petroleum Reserve to refineries to help alleviate any shortages caused by Hurricane Katrina, which ravaged oil and gas production facilities in the Gulf of Mexico. He said the Environmental Protection Agency also is temporarily waiving clean-air fuel standards nationwide, allowing the sale of larger quantities of gasoline and diesel fuel.

"This will help take some pressure off of gas price," Bush said. But he warned that "our citizens must understand this storm has disrupted the capacity to make gasoline and distribute gasoline."

Bush urged Americans to make private contributions -- in cash -- to the relief effort, noting that "the folks on the Gulf Coast are going to need the help of this country for a long time." He said, "This is going to be a difficult road. The challenges that we face on the ground are unprecedented. But there's no doubt in my mind we're going to succeed."

In time, he said, "New communities will flourish. The great city of New Orleans will be back on its feet. And America will be a stronger place for it."

In New Orleans, where tens of thousands of people remained despite a mandatory evacuation order issued Sunday, Mayor Ray Nagin said Katrina probably killed thousands. No confirmed casualty figures have yet been released for the city or affected parts of Louisiana.

"We know there is a significant number of dead bodies in the water," as well as other people dead in attics, Nagin said, according to the Associated Press. Asked how many dead, he said: "Minimum, hundreds. Most likely, thousands."

In Mississippi, which took a direct hit from Katrina, at least 110 people have been confirmed dead. But state officials have expressed concern that the toll would rise.

More than 1 million people remained without power in Gulf Coast states.

Nagin said that this time, there will be a "total evacuation" of New Orleans. He added, "We have to. The city will not be functional for two or three months."

He said the Superdome, crammed with 23,000 refugees in deteriorating conditions, "can no longer operate as a shelter of last resort." He estimated that 50,000 to 100,000 people stayed in the city of 485,000 despite the earlier evacuation order and said they would now be evacuated at the rate of 14,000 to 15,000 a day.

As federal and state officials and the American Red Cross made plans to empty the Superdome, sending the bulk of the refugees by bus to the Astrodome in Houston 350 miles away, the U.S. Army Corps of Engineers was organizing to try to plug two large breaches in floodwalls -- breaks that have allowed water from Lake Pontchartrain to flow into New Orleans, flooding 80 percent of the low-lying city.

Engineers planned to start by using helicopters to drop giant five-ton sandbags into the breaches. Once the floodwalls and levees protecting the city are repaired, the corps will work on pumping out the water, officials said.

In Washington, Chertoff told a news conference this afternoon that "we have had a number of breaches of levees." But he said that "the lake level has begun to decrease to some extent," which may cause some of the floodwater in the city to flow out. "Water may still be coming in, although it may be slowing," Chertoff said.

"We are now positioning the assets necessary to evacuate the Superdome," he said.

In the unusual news conference, attended by several other Cabinet secretaries, Health and Human Services Secretary Michael Leavitt said the Bush administration has declared a public health emergency for the whole Gulf Coast to help combat potential disease outbreaks. He said 10 federal medical shelters are being set up, and that 10 more will be established in the next few days.

Transportation Secretary Norman Y. Mineta said emergency supplies including 13.4 million liters of water are being shipped to the area and that the department is "working to restore at least minimum transportation infrastructure in the region," including oil pipelines.

"It's all hands on deck," Assistant Defense Secretary Paul McHale said. He said the Navy is moving eight ships to the area, including the hospital ship Comfort from Baltimore. He said the Comfort will depart Sept. 2 and arrive in the Gulf Coast region Sept. 8, and a 500-bed mobile hospital will likely be sent to the New Orleans area. The Pentagon also expects to provide a fleet of 50 helicopters to help assess damage and move federal relief personnel, McHale said.

Eight Navy SEAL swift-water rescue teams are being sent from California, and as many as 800 military personnel will assist the Red Cross with shelter support, he said. In addition, 11,000 National Guard troops are deployed in and around the affected area under the command of state governors.

Asked about widespread looting, McHale said this was a matter for civilian law enforcement agencies, possibly with National Guard units backing them up under the control of governors.

"In extraordinary circumstances," he said, the president has authority to use active-duty military personnel to restore order. There is no expectation that such deployments will be necessary, McHale said, but "we do have units that are on alert" and are prepared to be dispatched if called upon.

McHale said the National Guard "has a deep enough bench" to be effective, despite large deployments to Iraq. As of this morning, he said, 60 to 65 percent of the Louisiana and Mississippi National Guard forces were available for domestic duty, he said, adding, "A very robust capability remains within the affected states."