Authorities say the death toll from the devastating tornado outbreak across the southern U.S. has climbed to 318, making it the deadliest day for twisters since the Great Depression.

Alabama was in the path of the most destruction from Wednesday's storms. Authorities on Friday raised the number of confirmed dead in the state to 238. More than 30 lost their lives in Tuscaloosa, which is home to the University of Alabama. Two students are among the dead.

Alabama Gov. Robert Bentley said 1,700 people were injured by the tornadoes.

In March 1932, 332 people died, all in Alabama. In April 1974, a series of twisters killed 315 people in 11 states.

U.S. President Barack Obama his wife Michelle flew in to survey the damage Friday. The president expressed amazement at the destruction as he stepped through the wreckage in Tuscaloosa and pledged help to those who lost their homes in a terrifying flash.

"I've never seen devastation like this," he said. "We're going to make sure you're not forgotten."

'They're alongside God at this point'

As Obama and walked the streets of a reeling neighbourhood, he said that although nothing could be done for the many who were killed — "they're alongside God at this point" — Obama assured support for resilient survivors.

"What's amazing is when something like this happens folks forget all their petty differences," said the president after spending time talking to the state's governor and Tuscaloosa's mayor.

"When we're confronted by the awesome power of nature and reminded that all we have is each other."

Late Thursday, Obama signed a disaster declaration for the state to provide federal aid to those who seek it.

As many as a million homes and businesses there were without power, and Bentley said 2,000 National Guard troops had been activated to help. The governors of Mississippi and Georgia also issued emergency declarations for parts of their states.

The death toll spanned six states — more than two-thirds of them in Alabama — as America's deadliest tornado outbreak in almost four decades pulverized entire neighbourhoods.

Firefighters searched one splintered pile after another for survivors Thursday, combing the remains of houses and neighbourhoods of large cities that bore the one-kilometre-wide scars the twisters left behind.

The death toll from Wednesday's storms seems out of a bygone era, before Doppler radar and pinpoint satellite forecasts were around to warn communities of severe weather. Residents were told the tornadoes were coming up to 24 minutes ahead of time, but they were just too wide, too powerful and too locked onto populated areas to avoid a horrifying body count.

"These were the most intense super-cell thunderstorms that I think anybody who was out there forecasting has ever seen," said meteorologist Greg Carbin at the National Weather Service's Storm Prediction Center in Norman, Okla.

"If you experienced a direct hit from one of these, you'd have to be in a reinforced room, storm shelter or underground" to survive, Carbin said.

The storms seemed to hug the interstate highways as they barrelled along like runaway trucks, obliterating neighbourhoods or even entire towns from Tuscaloosa to Virginia. One family rode out the disaster in the basement of a funeral home, another by huddling in a tanning bed.

Bentley said his state had confirmed 210 deaths. There were 33 deaths in Mississippi, 33 in Tennessee, 15 in Georgia, five in Virginia and one in Kentucky. Hundreds if not thousands of people were injured — 600 in Tuscaloosa alone.

Some of the worst damage was in Tuscaloosa, a city of more than 83,000 that is home to the University of Alabama. The storms destroyed the city's emergency management centre, so the school's Bryant-Denny Stadium was turned into a makeshift one. School officials said two students were killed, though they did not say how they died. Finals were cancelled and commencement was postponed.

That twister and others Wednesday were several times more severe than a typical tornado, which is hundreds of yards wide, has winds around 160 km/h and stays on the ground for a few miles, said research meteorologist Harold Brooks at the Storm Prediction Center.

"There's a pretty good chance some of these were a mile wide, on the ground for tens of miles and had wind speeds over 200 mph [320 km/h]," he said.

The loss of life is the greatest from an outbreak of U.S. tornadoes since April 1974, when 329 people were killed by a storm that swept across 13 Southern and Midwestern states.

Massive storm

The U.S. National Weather Service says one of the tornadoes that killed hundreds in the South had winds of 402 km/h and was the first EF-5 tornado in Mississippi since 1966.

That's the highest rating given by the weather service for tornado damage. The weather service said Friday the tornado hit Smithville, Miss., at 3:44 p.m. ET on Wednesday, killing 14 and injuring 40.

The assessment is preliminary, based on photos taken Thursday and consultation with experts. It will be confirmed later this year after further inspections.

The EF-5 tornado in Smithville destroyed 18 homes, which the weather service said were well built, less than 10 years old and bolted to their foundation.