Devastating storms and tornadoes raked though the U.S. South, killing at least 185 people as they ripped houses to rubble, flipped cars and uprooted trees and power lines, officials said on Thursday.

In the deadliest series of storms in nearly four decades in the United States, 128 people were killed in Alabama, the worst-hit state.

In Mississippi, 32 were killed, while 10 people died in Georgia and 11 in Arkansas. Louisiana and Tennessee also reported deaths as the clusters of powerful tornadoes and storms tore a swathe of destruction from west to east.

Some of the worst devastation occurred on Wednesday in Alabama, where a massive mile (1.6 km)-wide tornado slammed into Tuscaloosa, home to the University of Alabama, killing at least 15 people, including some students.

Tuscaloosa resident Jannie Ross said she and her family took shelter in the basement of their home when the tornado struck.

"We could hear debris hitting the side of our house, glass breaking and the train sound often attributed to big storms such as these," she told the University of Alabama newspaper, The Crimson White.

"We could hear it destroying everything outside."

U.S. President Barack Obama declared a state of emergency for the state and ordered federal aid.

"Our hearts go out to all those who have been affected by this devastation and (we) stand ready to continue to help the people of Alabama," Obama said in a message on Twitter on Thursday.

The storms also forced the Tennessee Valley Authority to close three nuclear reactors at a power plant in Alabama and knocked out 25 high-voltage power lines.

Hundreds of thousands of homes have lost power.

"We have never experienced such a major weather event in our history," said the Tennessee Valley Authority, a U.S.-owned company that provides electricity to 9 million people in seven states.

Alabama Governor Robert Bentley declared a state of emergency and said he was deploying 2,000 National Guardsmen. Governors in Arkansas, Mississippi and Tennessee also declared states of emergency.

"We're in a search and rescue mode. We're making sure that those that may be out there that are trapped, that we have not found, we are trying to find them," Bentley told CNN.

"There has been massive devastation across northern Alabama. These long-track tornadoes really tear up the landscape as well as homes," he said.

DEVASTATING DAMAGE

Tornadoes are a regular feature of life in the U.S. South and Midwest, but they are rarely so devastating.

Images from Tuscaloosa, a town of around 95,000 in the west-central part of the state, showed widespread damage.

"Everybody says it (a tornado) sounds like a train and I started to hear the train," Anthony Foote, a resident of Tuscaloosa whose house was badly damaged, told Reuters.

"I ran and jumped into the tub and the house started shaking. Then glass started shattering."

The campus of the University of Alabama, home of the famous Crimson Tide football team, was not badly damaged but some students were killed off campus, Bentley said.

Damage in Alabama was spread over a wide area through the north and central part of the state, said Jennifer Ardis, Bentley's press secretary.

The highest toll was in Franklin County in the rural northwest part of the state where 18 people died, according to figures from the Alabama Emergency Management Agency.

Eleven people died in Jefferson County, home to Birmingham, the state's largest city, the agency figures showed.

Authorities in Alabama and Mississippi said they expect the death toll to rise as emergency workers attempt rescues and recovery in the storm's wake.