Some 75 million people were at risk from storms that could unleash hail, winds and twisters on the affected regions, according to the National Weather Service.
The toll from two days of violent weather reached at least 29 yesterday in at least six states, CNN and other media reported.
After violent weather on Sunday that killed 17, most of them in Arkansas, the new deaths included eight in Mississippi on Monday, CNN reported, quoting the state emergency management agency.
In the Mississippi town of Louisville, the storm snapped trees in half and stripped them of their branches. Sheet metal twisted itself around road signs and tree trunks, CBS News reported.
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Mississippi Senator Giles Ward hunkered down in a bathroom with his wife, four other family members and their dog Monday as a tornado destroyed his two-story brick house and flipped his son-in-law's SUV upside down.
"For about 30 seconds, it was unbelievable," Ward said. "It's about as awful as anything we've gone through," the network quoted Ward as saying.
In the hardest-hit parts of Arkansas, emergency crews intensified their search for survivors of Sunday's twisters.
In the town of Vilonia, police chief Brad McNew said the town of 4,000 had been rendered unrecognisable.
"It's houses completely down to the foundations," he told NBC television.
Rescuers used searchlights in blacked-out areas Sunday night, sifting through mountains of rubble in the hopes of finding someone alive.
The Arkansas Department of Emergency Management said 14 people had been killed in the state.
McNew said more would have been killed if not for emergency sirens that warned people the twister was about to hit.