At least 48 people have been killed and dozens injured by severe tornadoes in the southern United States.
The twisters hit several states including Arkansas, Mississippi and Tennessee at the end of the Super Tuesday presidential nominating votes.
A huge fire erupted at a natural gas pumping station in Tennessee that may have been damaged by the storms.
In the same state, several students were temporarily trapped in the wreckage of their dormitory.
The rare winter storms killed at least 24 people in Tennessee, 13 in Arkansas, seven in Kentucky and four in Alabama, authorities said.
President Bush offered disaster relief to those affected. "Prayers can help and so can the government," he said.
The US National Weather Service said dozens of tornadoes had touched down in an area overlapping three states.
The storms upturned vehicles, damaged buildings and uprooted trees. Hundreds of thousands of people are reported to have been left without power.
A couple and their 11-year-old daughter were killed when their home was destroyed by a tornado that touched down near the centre of the town of Atkins, Arkansas.
Pope County Coroner Leonard Krout said their home "took a direct hit".
"Neighbours and friends who were there said, 'There used to be a home there'," Mr Krout told the Associated Press news agency (AP).
Rescuers picked their way past downed power lines to search the town in the dark for survivors.
"This was an extraordinary night," said Arkansas Governor Mike Beebe. "When it's compounded by darkness, that makes it that much more difficult."
A couple were killed with their adult daughter in their mobile home near Greenville, in western Kentucky, when a tornado went through a trailer park.
Four more people were killed in Allen County, near the state border with Tennessee, a spokesman for the Kentucky Division of Emergency Management told AP.
Flames shot hundreds of feet in the air after the Columbia Gulf Natural Gas pumping station in Hartsville, Tennessee, caught fire.
Officials said it had been struck by a tornado.
A nearby resident said her home had been flattened by the storm.
At least three people died in Atkins, Arkansas
"It just took the house and everything and my horses and my dog," Dara Reasonover told CNN. "I don't know if they're alive or dead, but we'll make it."
At Union University in Jackson, western Tennessee, several students were freed after being trapped in their badly damaged dormitory.
Union University President David Dockery told US media that 51 students had been taken to the hospital, and nine had stayed overnight.
'Like a bomb'
In Memphis, Tennessee, high winds collapsed the roof of a Sears store at a shopping mall.
Police said some people who took shelter under a nearby bridge were washed away, but were pulled from the Wolf River with only minor injuries.
A shopping centre in Tennessee was severely damaged
In Southaven, Mississippi, a tornado wrecked warehouses in an industrial park.
Local police commander Steve Atkinson said: "It ripped the warehouses apart. The best way to describe it is it looks like a bomb went off."
Presidential candidates including Hillary Clinton, Barack Obama and Mike Huckabee interrupted their Super Tuesday speeches to express sympathy for the victims.
There were numerous severe weather warnings on TV but i did not think it would get this bad. For most of the day the weather was normal but around 8pm the wind started blowing ferociously. All of a sudden we could hear sirens and we were ordered to the ground floor of our dormitory. After about half an hour it quickly became calm again. Luckily for us Arkansas State University was not damaged.
Macnos Mutano, Jonesboro, USA
We are in Starkville, Mississippi and are getting hit by the severe weather really bad right now and I'm surprised that we still have power and internet capabilities. It is unknown how bad the aftermath of the storm will be until the sun rises. We are still fearful of tornadoes here. Please keep us in your intentions.
Trevor Caulkins, Starkville, Mississippi USA
I live in Jackson, Tennessee, and watched the tornado as it passed less than a half-mile from where I was standing. There are 8 students still trapped in the University, as one of the dormitories has collapsed. As well as 50 elderly trapped inside a retirement home that collapsed. It has definitely been a terrifying night. Trees, buildings, power lines, street lights...they are down all over the place.
Meaghan, Jackson, Tennessee, US
A friend's mother was hiding in a bathtub in Jackson, TN. We have not heard from her as of this time because phone lines are down. This is not the first time for tornadoes to rip through that area. The Main Street was destroyed a few years back in the same town.
Vloxy, Dallas, TX