At least 35 people have been killed in tornadoes that reached up to 320km/h, ravaging neighbourhoods across six states, US media say.
In hard-hit Mississippi, Alabama and Tennessee, at least 17 people were killed Monday, CNN television said. Another 18 were reported dead in Arkansas, Iowa and Oklahoma on Sunday.
But forecasters warned more brutal weather was set to hit, affecting an estimated 70 million people.
The National Weather Service said that "severe thunderstorms" were expected in eastern and southern Mississippi, western Alabama and extreme eastern Louisiana.
It also forecast several tornadoes, some of them "intense," baseball-sized hail and damaging winds.
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Among the dead was University of Alabama student John Servati, who perished saving his girlfriend from a wall that fell and would have otherwise crushed her, The Clarion-Ledger reported.
"I called my coach when I found out this morning; we just kind of balled for a moment," high school friend Colton Fremont said.
"It was surreal to hear one of my best swimming friends I've had my entire life is gone just like that."
People in Tupelo, Mississippi sifted through the rubble of their destroyed homes and businesses.
Some grilled hotdogs and hamburgers to hand out for free to those in need, while youths helped remove debris and fallen tree limbs from elderly people's homes and yards.
"I am just overwhelmed -- the damage is overwhelming, of course, but the outpouring of people to lend a hand and give out water and food, it's just tremendous," said Denise Hardin of the Tupelo Housing Authority that manages many neighbourhood properties.
