"It took the whole house," one woman in Tennessee told CNN news channel.
"I just got up and I thought it was a fire. We got up and there was no house left."
Tornado watches were in effect on Wednesday morning for parts of
Alabama, Tennessee, Georgia and the western Florida panhandle, the
National Weather Service said.
In Tennessee, twisters knocked down a police radio tower, crushed the wall of a shopping centre and damaged a hangar at the airport in the largest city, Memphis.
"We're still trying to assess all the damage now that the sun is
coming up," Laura McPherson of the Tennessee Emergency Management Area told AFP news agency.
Several people sheltering under a bridge north of the shopping centre were washed into state's Wolf river, but were rescued, Steve Cole of the Memphis police department said.
The same storm then moved to Jackson, Mississippi, and damaged a dormitory at Union University, where eight students were temporarily trapped but escaped serious injury, school officials said.
At least six tornadoes occurred between Oxford, Mississippi, and Jackson, Tennessee, alone, Richard Okulski, from the National Weather Service in Memphis, said.