The death toll from the devastating tornadoes that hit America's south climbed to 337 today, making it the second-deadliest day for a twister outbreak in US history.

The largest death toll was on March 18, 1925, when 747 people were killed in a line of storms that raged through Missouri, Illinois and Indiana.

Alabama was in the path of the most destruction this time with at least 246 deaths. Authorities raised the total number of confirmed dead in several states to 337 today.

The second deadliest day had been in March 1932, when 332 people died, all in Alabama.

The 1925 outbreak was long before Doppler radar could warn communities of severe weather. Forecasters say residents were warned, but the twisters were too powerful to avoid the horrifying body count.

Hundreds if not thousands of people were injured on Wednesday - 990 in Tuscaloosa alone - and as many as one million Alabama homes and businesses remained without power.

The scale of the disaster astonished President Barack Obama when he arrived in the state yesterday.

"I've never seen devastation like this," he said, standing in bright sunshine amid the wreckage in Tuscaloosa, where at least 45 people were killed and entire neighbourhoods were flattened.

Hours later, Mr Obama signed disaster declarations for Mississippi and Georgia, in addition to one he had authorised for Alabama.

Tuscaloosa Mayor Walt Maddox called the devastation "a humanitarian crisis" for his city of more than 83,000.

He said up to 446 people were unaccounted for in the city, though he added that many of those reports were probably from people who have since found their loved ones but have not notified authorities. Cadaver-detecting dogs were deployed in the city yesterday but they had not found any remains, he added.

During the mayor's news conference, a man asked him for help getting into his home, and broke down as he told his story.

"You have the right to cry," Mr Maddox told him. "And I can tell you, the people of Tuscaloosa are crying with you."

Last night, Tuscaloosa officials reduced the death toll for the city and its police jurisdiction by six to 39, still the most in Alabama. With that change factored in, the state's death toll stood at 246 early today.

At least one tornado - a 205mph (330kph) monster which left at least 13 people dead in Smithville, Mississippi - ranked in the National Weather Service's most devastating category, EF-5.

Meteorologist Jim LaDue said he expected "many more" of Wednesday's tornadoes to receive that same rating, with winds topping 200mph (322 kph).

Tornadoes struck with unexpected speed in several states, and the difference between life and death was hard to fathom. Four people died in Bledsoe County, Tennessee, but a family survived being tossed across a road in their modular home, which was destroyed, Mayor Bobby Collier said.

By yesterday, residents whose homes were blown to pieces were seeing their losses worsen - not by nature, but by man. In Tuscaloosa and other cities, looters have been picking through the wreckage to steal what little the victims have left.

"The first night they took my jewellery, my watch, my guns," Shirley Long said. "They were out here again last night doing it again."

Overwhelmed Tuscaloosa police imposed a curfew and got help from National Guard troops to try to stop the scavenging.

Along their flattened paths, the twisters blew down police and fire stations and other emergency buildings along with homes, businesses, churches and power infrastructure. The number of buildings lost and people left homeless remained unclear two days later, in part because the storm also ravaged communications systems.

Tuscaloosa's emergency management centre was destroyed, so officials used space in one of the city's most prominent buildings - the University of Alabama's Bryant-Denny Stadium - as a substitute before moving operations to the Alabama Fire College. Less than two weeks ago, the stadium hosted more than 90,000 fans for the football team's spring intra-squad Red-White Game.

A fire station was destroyed in nearby Alberta City, one of the city's worst-hit neighbourhoods. The firefighters survived, but damage to their equipment forced them to begin rescue operations without a fire truck, Fire Chief Alan Martin said.

Mr Martin said the department was running normally and has since deployed a back-up vehicle to serve the neighbourhood. "In reality, it's just an extension of what we do every day," he said.

Also wiped out was a Salvation Army building, costing Tuscaloosa much-needed shelter space. And that is just part of the problem in providing emergency aid, said Sister Carol Ann Gray, of the local Catholic Social Services office.

"It has been extremely difficult to co-ordinate because so many people have been affected - some of the very same people you'd look to for assistance," she added.

Emergency services were stretched particularly thin about 90 miles (145km) to the north in the demolished town of Hackleburg, Alabama, where officials were keeping the dead in a refrigerated truck amid a body bag shortage. At least 27 people were killed there and the search for missing people continued, with FBI agents fanning out to local hospitals to help.

Damage in Hackleburg was catastrophic, said Stanley Webb, chief agent in the county's drug task force.

"When we talk about these homes, they are not damaged. They are gone," he said.

Gail Enlow was in town looking for her aunt, Eunice Cooper, who is in her 70s. She wiped away tears as she pointed to the twisted mess which is all that is left of the housing project where Ms Cooper lived.

"Nobody's seen her," she said, trying to hold back the sobs. "She can just barely get around and she would need help."

In Hackleburg, as in Tuscaloosa, emergency workers had more to do than aid suffering victims. People looted a demolished Wrangler jeans distribution centre, and authorities locked up drugs from a destroyed pharmacy in a bank.

Fire Chief Steve Hood said he desperately wanted torches for the town's 1,500 residents because he does not want them using candles which could set light to their homes.

In Cullman, a town about 50 miles (80km) north of Birmingham, workers have been putting in long hours to clean up debris and exhausted police officers face the same problems as the people they are sworn to protect. Emergency responders have waiting in queues with other drivers for hours to get fuel at stations without power.

False rumours, meanwhile, were sweeping the town. People were pushing debris from their yards into streets because they heard they were supposed to and filling up their bathtubs with water because they heard the city would cut off the supply.

Kathy McDonald glanced around her damaged town and quietly wept. Her family's furniture store, which sold tables and couches for decades, was torn apart.

"I just can't understand this. Are people coming to help us?" she said. "We feel all alone."

Other states were reeling as well. There were 34 deaths in Mississippi, 34 in Tennessee, 15 in Georgia, five in Virginia, two in Louisiana and one in Kentucky.

The Federal Emergency Management Agency has responded to all affected areas and has officials on the ground in Alabama, Mississippi, Kentucky, Georgia and Tennessee, director Craig Fugate said. State and local authorities remain in charge of response and recovery efforts, he added.

In the Birmingham suburb of Pleasant Grove, where 10 people died, building contractors used heavy equipment to help clear debris from impassable streets yesterday.

Volunteers arrived from as far as Mobile - some 250 miles (400km) away - to deliver food, water and fuel and help with search and rescue. The National Guard closed the town to outsiders, trying to keep out gawkers and looters.

Police Chief Robert Knight said perhaps a quarter of the town of 10,000 is wiped out.

"We're having a hard time recovering," he said. But he vowed that residents would rebuild.

"We'll do it. We'll do it," he said. "We just will. People out here are resilient. It's a good city."