TUSCALOOSA, Ala. - Tornadoes and violent storms ripped through seven Southern states, killing at least 295 people and causing billions of dollars of damage in some of the deadliest twisters in US history.

President Barack Obama described the loss of life as "heartbreaking" and called the damage to homes and businesses "nothing short of catastrophic." He promised strong federal support for rebuilding.

Over several days this week, the powerful tornadoes -- more than 160 reported in total -- combined with storms to cut a swath of destruction heading west to east. It was the worst US natural disaster since Hurricane Katrina in 2005, which killed up to 1,800 people.

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In some areas, whole neighborhoods were flattened, cars flipped over and trees and power lines felled, leaving tangled wreckage.

While rescue officials searched for survivors, some who sheltered in bathtubs, closets and basements told of miraculous escapes. "I made it. I got in a closet, put a pillow over my face and held on for dear life because it started sucking me up," said Angela Smith of Tuscaloosa, one of the worst-hit cities.

Tornadoes are a regular feature of life in the US South and Midwest, but they are rarely so devastating.

Wednesday was the deadliest day of tornadoes in the United States since 310 people lost their lives on April 3, 1974.

Given the apparent destruction, insurance experts were wary of estimating damage costs, but believed they would run into the billions of dollars, with the worst impact concentrated in the Alabama cities of Tuscaloosa and Birmingham.

"In terms of the ground-up damage and quite possibly the insured damage, this event will be of historic proportions," Jose Miranda, an executive with the catastrophe risk modeling firm EQECAT, told Reuters.

'Major, major disaster'

Overnight tornadoes leaves part of Pratt City, a suburb of Birmingham, Alabama, in ruins April 28, 2011. [Photo/Agencies] More Big Photos

"We have right now 194 fatalities (in Alabama) that we know," state Governor Robert Bentley told a news conference.

"That is a major, major disaster," said Bentley, who earlier also reported "massive destruction of property."

"I've never seen devastation like this in Alabama," said Alabama Senator Richard Shelby, who flew over storm-hit areas of the state by helicopter on Thursday. "I've seen pictures of hurricanes and so forth, but this is the worst I've seen."

In preliminary estimates, other states' officials reported 32 killed in Mississippi, 34 in Tennessee, 11 in Arkansas, 14 in Georgia, eight in Virginia and two in Louisiana.

The mile (1.6 km)-wide monster twister that tore on Wednesday through Tuscaloosa, home to the University of Alabama, may have been the biggest ever to hit the state, AccuWeather.com meteorologist Josh Nagelberg said.

Obama said he would visit Alabama on Friday to view damage and meet the governor. Obama declared a state of emergency for Alabama and ordered federal aid.

"I want every American who has been affected by this disaster to know that the federal government will do everything we can to help you recover, and we will stand with you as you rebuild," Obama said at the White House.

Miranda said estimated costs would be "in the same ballpark" as an Oklahoma City tornado outbreak in 1999 that caused $1.58 billion of damage and a 2003 tornado outbreak in Kansas, Missouri, Arkansas and Oklahoma that caused $4.5 billion of damage.

"I would not be surprised to see it in the mid-level billions, singular billions, of dollars," Miranda said.

The Browns Ferry nuclear power plant in Alabama was expected to be shut for days, possibly weeks, as workers repaired damaged transmission lines.

But the backup systems worked as intended to prevent a partial meltdown like the nuclear disaster in Japan.

The rampaging tornadoes and violent storms destroyed 200 chicken houses that held up to 4 million chickens in Alabama, the No. 3 US chicken producer.

They also battered a local coal mine.

Up to 1 million people in Alabama were left without power.

Daimler said it had shut down its Mercedes-Benz vehicle assembly plant in Tuscaloosa until Monday due to the tornadoes, but the plant itself sustained only minor damage.

'Sounded like chain-saw'

The aftermath of overnight tornadoes show destroyed homes and vehicles in Pratt City, a suburb of Birmingham, Alabama, April 28, 2011.[Photo/Agencies] More Big Photos

Some of the worst devastation occurred in Tuscaloosa, a town of about 95,000 in the west-central part of Alabama, where at least 37 people were killed, including some students.

"It sounded like a chain-saw. You could hear the debris hitting things. All I have left is a few clothes and tools that were too heavy for the storm to pick up. It doesn't seem real," said student Steve Niven, 24.

"I can buy new things but you cannot replace the people. I feel sorry for those who lost loved ones," Niven told Reuters.

The campus of the University of Alabama, home of the famous Crimson Tide football team, was not badly damaged, but some students were killed off campus, Bentley said.

Shops, shopping malls, drug stores, gas stations and dry cleaners were all flattened in one section of Tuscaloosa.

Alabama's governor declared a state of emergency and deployed 2,000 National Guard members. Arkansas, Mississippi, Tennessee and Virginia also declared states of emergency.

Among the Alabama counties affected was Jefferson, which is struggling to avert what would be the largest bankruptcy in municipal history over a $3.2 billion bond debt.

The county suffered "widespread damage," a local emergency spokesman said, and at least 17 people were killed.