BIRMINGHAM, Alabama — Tornadoes and storms carved a trail of "catastrophic" destruction across the southern United States, killing over 250 people in the worst US weather disaster in years, officials said Thursday.

Shocked residents of cities and towns in more half a dozen states crippled by the ferocious spring storms picked through the remains of their destroyed homes, businesses, schools and churches, in surreal scenes of devastation more common to war zones and massive earthquakes.

The severe weather killed 162 people on Wednesday in Alabama alone, authorities told AFP, and the White House announced that President Barack Obama would travel to the state on Friday for a first-hand look at the devastation.

Obama had earlier said Washington was rushing federal assistance to the battered state along the Gulf Coast.

Emergencies were declared in Alabama, Arkansas, Georgia, Illinois, Kentucky, Mississippi, Missouri, Oklahoma and Tennessee, and governors called out the National Guard -- including 2,000 troops in hardest hit Alabama -- to help with rescue and cleanup operations.

"We had a major catastrophic event here in Alabama with the outbreak of numerous long-track tornadoes," said Alabama Governor Robert Bentley said, who declared a major disaster for the state and estimated that up to a million in his state were without power.

"We do have major destruction," Bentley said.

In the region, officials reported at least 258 people dead, but as the residents and emergency workers began to mop up and assess the damage the toll was likely to rise.

"In fact, we're sure it will," Bentley said, echoing concerns of officials across several states in what is being described as the worst US tornadoes since 310 people were killed on April 3, 1974.

The National Weather Service (NWS) had preliminary reports of more than 300 tornadoes since storms began Friday, including more than 130 on Wednesday alone.

Alabama was slammed by two lines of storms and an evening tornado that tore through the city of Tuscaloosa, home to the University of Alabama.

Tuscaloosa Mayor Walter Maddox told CNN the tornado had "obliterated blocks and blocks" of his city, leaving 36 people dead there.

"Infrastructure has been absolutely devastated," he said Thursday. "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.

"There are parts of this city I don't recognize," he added.

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. "I'm not talking about just roofs being blown off, but just completely gone."

The area, he told National Public Radio, "seems like a bomb has been dropped on it."

Incredulous Birmingham residents assessing the damage were counting their blessings to still be alive, while others were distraught over the loss of their loved ones or homes.

"There were two-by-fours (wooden beams) falling out of the sky," convenience store manager Jack Welch said, adding "there were well over 30 homes destroyed" just behind his store.

In Mississippi storms killed 32 people and caused damage in 50 of its 82 counties, while state officials reported at least 30 dead in Tennessee.

Thirteen died in Georgia, 11 in Arkansas, eight in Virginia, and another two killed in severe flooding in Missouri, state officials said.

"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."

"It's an 80 percent loss in our town," he added.

Storm victims across the region were trapped in homes, trailers and cars by falling trees. Hail the size of golf balls cracked windows.

Roads were washed out or blocked by fallen trees and power lines across the region. Homes, schools and businesses were flattened, flooded and set on fire by lightning.

The storms come after a wet spring and a winter of heavy snowfall which has left the ground saturated and rivers running high.

The NWS warned that severe weather could strike 21 states from the Great Lakes down to the Gulf Coast and across to the Atlantic, and tornadoes were reported as far east and north as Maryland, bordering the capital Washington.

Another major storm system is forecast to bring heavy rain and high winds on Saturday.