Authorities along the battered Gulf Coast, handicapped by impassable roads and outages of power and communication, began the massive mission Tuesday of finding stranded survivors of Katrina, providing assistance to those made homeless and assessing the damage over a 500-mile stretch of Louisiana, Mississippi and Alabama.

Louisiana officials announced at an afternoon news conference that the situation in New Orleans was deteriorating because of breaches in the system of levees that protect the city, which is lower than sea level. They said that after rescue operations have been completed, they hope to evacuate those people still left in the city.

Even by late Tuesday afternoon, good information about the extent of the damage was hard to come by, officials said. Coast Guard officials in Louisiana reported rescuing several hundred people from rooftops Monday night and Tuesday morning but could offer no estimate of the numbers still requiring rescue. Plus, many smaller coastal communities had not been heard from.

While Mississippi Gov. Haley Barbour (R) said Tuesday morning that there were unconfirmed reports of up to 80 fatalities in Mississippi, the number of dead could not be established. There were reports that as many as 30 people may have died when the 100-unit Quiet Waters Beach apartments in Biloxi collapsed, but there was no official confirmation.

Nobody, however, questioned Barbour's statement that "the devastation down there is just enormous."

And few doubted that Hurricane Katrina is likely to go down in history as one of the worst natural disasters in the United States.

It is "very, very sobering," Michael D. Brown, head of the Federal Emergency Management Agency, said on CBS's "The Early Show." "The flooding is just everywhere . . . New Orleans, all through Mississippi and Alabama. The storm is really having a catastrophic effect."

He told the Associated Press that tens of thousands of people will need shelter for weeks, if not months and warned that even after the floodwaters recede, "it's going to be incredibly dangerous" throughout the region because of widespread structural damage, chemicals in homes and environmental damage.

Fears were also growing about pollution, since the water was believed to be carrying sewage, spilled fuel and other pollutants.

In New Orleans, where officials only Monday had been hoping that they avoided the worst of the storm, faced significant new hurdles Tuesday. Water from Lake Pontchartrain to the north coursed through lower-lying streets, released by a breached levee that normally protects the city. Water was rising in some areas of downtown.

There also were reports of natural gas leaks and some hospitals lacked power.

Officials and television reports suggested numerous incidents of looting in New Orleans and Biloxi.

In Biloxi, the sun rose on deserted streets coated with muck and littered with downed trees and abandoned trucks. Utility poles were leaning every which way, and severed power and phone lines hung limply everywhere.

"Miss Marie lost her life," said despondent Biloxi resident Lillian Pritchett of an elderly neighbor who could not make it out of her house. "She drowned. She couldn't get out. . . .

"They need to start sending somebody through here," she said. "We need ice. We need water. We need food."

Help was on the way in Mississippi. Contingents of National Guard troops, police and other state officials armed with chainsaws were indeed the only people on the road leading south from Jackson. They were allowing only emergency service workers into Biloxi.

The Red Cross was sending 185 emergency vehicles to provide meals and services throughout the region.

"This is the largest mobilization of Red Cross resources" ever for a storm, said agency official Pat McCrummen.

Officials across the impacted area remained uncertain and deeply concerned about what they would find in their search-and-rescue operations just underway and they were severely hampered in their communication.

"We've got no power down there and no telephone service," Heath Carpenter of Mississippi's emergency management agency said on TV of the more remote coastal areas of the state.

More than 1 million people throughout the region are without power, officials said. Tens of thousands are unable to enter their homes. Untold numbers of homes are uninhabitable or totally destroyed. Shelters are full.

TV footage showed many areas still flooded, apartment buildings and houses collapsed or thrown off their foundations, roads littered with trees, debris and overturned vehicles and roadways and causeways broken apart.

In Biloxi, waterfront casinos, the engine of the local economy, were crushed and hundreds of homes were damaged. Vincent Creel, a spokesman for the city, said he expected the number of deaths here "is going to be in the hundreds," the Associated Press reported.

In Louisiana, officials said people in some swamped neighborhoods were feared dead, but gave no immediate numbers, wire services reported.

"All I know is when my people go out, they tell me there are a lot of people awaiting rescue. I hear there are hundreds of people still on their rooftops," said Gen. Ralph Lupin, commander of National Guard troops at the Superdome.

Katrina, "downgraded" to a tropical depression the afternoon and weakening, nevertheless continued to threaten points north and east of the coast in Tennessee, Kentucky and Ohio as she traveled away from the scene of the worst damage, bearing heavy rain, 40 to 50 mph winds and possibly tornadoes.

Fred Barbash and Daniela Deane reported from Washington. Whoriskey reported from New Orleans.