TUSCALOOSA, Alabama (AFP) – Shocked Americans on Friday sifted through the rubble from the worst US tornadoes in decades, which carved a trail of destruction across the south claiming at least 313 lives.

Communities like Alabama Governor Robert Bentley's home town of Tuscaloosa were virtually wiped off the map, and officials warned the body count would rise as rescuers uncovered more dead in the debris.

Disbelief was written on faces across eight states crippled by the ferocious spring storms -- the deadliest tornado tragedy to strike the United States since 332 people were killed by a tornado outbreak on March 21, 1932.

Recalling the more recent horror of Hurricane Katrina in 2005, families picked through the remains of homes, businesses and schools, bearing witness to scenes of devastation more common in war zones or after earthquakes.

In Alabama, the worst-hit state, the toll reached 210, with more than 1,700 injured and up to a million people left without power. The Birmingham News quoted local officials as saying it could take days to restore electricity.

US President Barack Obama and First Lady Michelle were to travel to the state on Friday for a first-hand look at a still unfolding human tragedy.

"The loss of life has been heartbreaking, especially in Alabama," Obama said, describing the disaster as "nothing short of catastrophic."

Obama has declared a "major disaster" in Alabama and ordered federal aid to assist state and local recovery efforts, including grants for temporary housing and home repairs and loans to cover uninsured property damage.

States of emergency were declared from central Oklahoma to Georgia on the eastern seaboard, and governors called out the National Guard -- including 2,000 troops in Alabama -- to help with the rescue and clean-up operations.

"We had a major catastrophic event here in Alabama with the outbreak of numerous long-track tornadoes," said Governor Bentley.

In neighboring Mississippi, which suffered more than 30 casualties, Governor Haley Barbour told CNN that Wednesday "was just as bad as I can ever remember. Some people will make the argument it is as bad as it has ever been."

As the long day dragged on, rescue workers battled to find missing people and try to rescue survivors still trapped in the rubble of their homes.

Many homes looked like they had been blown inside out, with the walls torn down and furniture spilling into the street.

In a parking lot at the University of Alabama in Tuscaloosa -- where 36 people were confirmed dead -- tornadoes left behind 20 smashed cars, many of them piled on top of one another.

"I don't want to think now in how much I lost," Robert Mitton told AFP. "I hope we can get some help from the government. I live very close, my house is very damaged, but my family is fine."

Owen Simmons, who works in a furniture store, pointed to a black cross and a zero below painted on the side of his house.

"It means that the rescue team has already checked my home and they found no victims. That's what really matters."

Tuscaloosa Mayor Walter Maddox said his town's infrastructure had been completely devastated.

"When you look at this path of destruction, likely five to seven miles (eight to 11 kilometers) long and half a mile to a mile wide, I don't know how anyone survived," he told CNN. "There are parts of this city I don't recognize."

It was also a dark day for Birmingham, Alabama's largest city with more than a million residents. Mayor William Bell spoke of "whole neighborhoods of housing, just completely gone. Churches, gone. Businesses, gone."

The overall toll includes 34 deaths in Tennessee, 33 in Mississippi, 15 in Georgia, 13 in Arkansas, five in Virginia, two in Missouri, and one in Kentucky, according to state officials.

"Oh my God, our town is in pieces," said Tim Holt, a clerk at a local hotel in Ringgold, Georgia. "We saw the funnel cloud coming and I ran into the bathroom with my wife and daughter."

Violent twisters that famously rip through the US south's "Tornado Alley" are formed when strong jet winds bringing upper-level storms from the north interact with very warm, humid air mass from the Gulf of Mexico.

The record for confirmed tornadoes over a 24-hour period is 148, set during the so-called "Super Outbreak" in 1974. More than 160 twisters were reported on Wednesday but not yet confirmed.

The tornado disaster is already the fourth worst on record in the United States and when the final toll is known it may only be surpassed by the giant Tri-State Tornado of March 1925, which left 747 people dead.