The death toll from severe storms that roared across the U.S. South has risen to at least 290 across six states following the worst outbreak of tornadoes in the U.S. in almost 40 years.

Alabama Gov. Robert Bentley told a news conference Thursday afternoon that at least 194 people were known to have died in his state. His office subsequently confirmed 10 additional deaths.

There were also at least 33 deaths in Mississippi, 33 in Tennessee, 14 in Georgia, eight in Virginia and one in Kentucky.

Other reports put the toll across the six states as high as 304.

The National Weather Service's Storm Prediction Center in Norman, Okla., said it received 137 tornado reports around the regions into Wednesday night.

The fierce storms spawned tornadoes and have wiped out homes and businesses, forced a nuclear power plant to use backup generators and even prompted the evacuation of a National Weather Service office.

In Huntsville, Al., meteorologists found themselves in the path of severe storms and had to take shelter in a reinforced steel room, turning over monitoring duties to a sister office in Jackson, Miss. Meteorologists saw multiple wall clouds, which sometimes spawn tornadoes, and decided to take cover, but the building wasn't damaged.

"We have to take shelter just like the rest of the people," said meteorologist Chelly Amin, who wasn't at the office at the time but spoke with colleagues about the situation.

Dave Imy, a meteorologist with the National Weather Service's Storm Prediction Center in Norman, Okla., said the deaths were the most in a tornado outbreak since 1974, when 315 people died.

Tuscaloosa hard hit

One of the hardest-hit areas Wednesday was Tuscaloosa, a city of more than 83,000 and home to the University of Alabama. The city's police and other emergency services were devastated, the mayor said, and at least 15 people were killed and about 100 were in a single hospital.

A massive tornado, caught on video by a news camera on a tower, barrelled through the city late Wednesday afternoon, levelling it.

By nightfall, the city was dark. Roads were impassable. Signs were blown down in front of restaurants, businesses were unrecognizable and sirens wailed off and on.

People reclaim some of their belongings Thursday after a tornado struck Tuscaloosa, Ala., the day before. David Bundy/Associated Press

Piles of rubble lined the sidewalks. A woman, too distraught to talk, staggered down the street.

"I can't find my husband," she cried, cupping her hands to her face.

Alabama student Kristin Wolfe carried an injured dog down the middle of University Boulevard, the neighbourhood's main drag, after digging the animal out of some rubble. The dog's head rolled in her arms, and it growled weakly when someone approached to try and give it water.

"I don't think he's going to make it," she said.

Within moments, the animal died. Wolfe and some Air Force ROTC cadets left it by the side of the road, unable to do anything more.

College students in a commercial district near campus used flashlights to check out the damage.

At Stephanie's Flowers, owner Bronson Englebert used the headlights from two delivery vans to see what valuables he could remove. He had closed early, which was a good thing. The storm blew out the front of his store, pulled down the ceiling and shattered the windows, leaving only the curtains flapping in the breeze.

"It even blew out the back wall, and I've got bricks on top of two delivery vans now," Englebert said.

A group of students stopped to help Englebert, carrying out items like computers and printers and putting them in his van.

"They've been awfully good to me so far," Englebert said.

Tuscaloosa Mayor Walter Maddox told reporters that police and the National Guard will impose a curfew at 10 p.m. Thursday,

U.S. President Barack Obama, speaking at the White House, called the tornado damage "nothing short of catastrophic."

Obama will tour the worst-hit areas of Alabama on Friday.