250 die in one day as twisters rampage from Texas to Virginia

Search-and-rescue teams were last night hunting for survivors beneath fallen masonry and tangled joists, power lines and fallen trees in towns and hamlets across seven states in the south-eastern US which were ravaged by the worst outbreak of tornadoes seen in almost four decades. Almost 250 people are known to have died.

President Barack Obama, promising help in rebuilding, described the loss of life as "heartbreaking" and called the damage to homes and businesses "nothing short of catastrophic".

Stunned residents of Tuscaloosa in Alabama awoke yesterday to discover a city partly razed by a single killer twister that cut a swath more than a mile wide and may have got close to an F5 level, the highest on the intensity scale. Shopping centres, shops and whole residential neighbourhoods were shredded, reduced to an almost unfathomable jumble of destruction. At least 169 people died in Alabama alone.

Tornado swarms are not uncommon across the central belt of the US in the spring, when cold air does battle with much warmer and humid air pushing up from the Gulf of Mexico. But not since 1974, when a single outbreak killed 315 people, has twister activity been as high as this year.

The death toll from Wednesday's super-outbreak, which saw tornado touchdowns along a corridor from Texas to Virginia, stood at 249 and officials said it was likely to rise further as new victims were dug from destroyed homes and businesses. Avalanche rescue dogs were deployed in some areas to seek out victims.

Among the affected states, the worst of the damage appeared to be in Alabama. Those killed in Tuscaloosa, home to the University of Alabama, stood at 15, while hundreds of others suffered injuries. Meanwhile, a nuclear power plant at Browns Ferry, to the west of Huntsville, was knocked off line with safety power coming only from back-up generators. Power officials said it might be weeks before the plant is back on line.

This tornado season may be one of the deadliest on record even though modern forecasting and communications technology, including social networking sites such as Twitter and Facebook, has made it easier to issue general warnings than was the case a few years ago.

But storms such as the ones that raked through the region on Wednesday can sometimes simply be too fierce, gouging everything they touch. An F5 will destroy a brick building down to its foundations, while felling trees and lifting vehicles clean off the road before dumping them elsewhere. In those conditions, there is nowhere for people to hide if they are in its path.

"Everybody says it sounds like a train and I started to hear the train," said Anthony Foote, whose Tuscaloosa home was badly damaged. "I ran and jumped into the tub and the house started shaking. Then glass started shattering."

"You cannot prepare against an F5," said the Governor of Alabama, Robert Bentley, who in common with some neighbouring states declared a state of emergency. President Obama promised federal assistance.

Officials said the University of Alabama campus was largely unscathed, but that did not mean students living elsewhere in Tuscaloosa were not among the dead. "It sounded like a chainsaw," said student Steve Niven, 24, who lives off-campus. "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. I can buy new things but you cannot replace the people."