Emergency crews have been digging through the rubble for survivors of deadly tornadoes that ripped through several central and southern states of the US.
At least 17 people died in Arkansas and Oklahoma - 16 of them in the suburbs of Little Rock, Arkansas.
Tornadoes also struck in Nebraska, Kansas, Iowa and Missouri.
Forecasters have warned millions of people to prepare for more severe storms in the region on Monday.
There are warnings of further tornadoes, high winds and large hailstones to strike parts of Mississippi, Alabama, Tennessee and Louisiana.
"We've got a powerful storm system affecting the eastern two-thirds of the United States over the next few days,'' said Russell Schneider, director of the Storm Prediction Center in Norman, Oklahoma.
'Mass casualties'
Authorities said the death toll from Sunday may rise as crews searched the wreckage of destroyed buildings.
"We're trying to make sure everyone is accounted for," Brandon Morris, spokesman for the Arkansas Department of Emergency Management, told the Associated Press news agency.
Arkansas Governor Mike Beebe said the storm "may be one of the strongest we have seen".
"We don't have a count on injuries or missing. We're trying to get a handle on the missing part," he said.
A trailer was overturned in Mayflower, Arkansas 
People searched the rubble following a tornado strike in Vilonia, Arkansas 
Parts of Baxter Springs, Kansas, were also damaged by the dangerous storms President Barack Obama, on a trip to the Philippines, offered his deepest condolences to those affected on Sunday and said federal emergency officials would be on the ground to help.
"Your country will be there to help you recover and rebuild, as long as it takes," he said.
Most of the casualties were in suburbs west and north of Little Rock. The Arkansas Department of Emergency Management said 10 people died in Faulkner County, five in Pulaski County and one in White County.
Building were destroyed while vehicles and caravans were thrown into the air
Mayflower and Vilonia, two small towns in Faulkner County, appear to have borne the brunt of the damage. In Vilonia, the County Sheriff's office spoke of a "mass casualty situation".
The Arkansas tornado touched down about 10 miles (16km) west of the city of Little Rock and left a 40-mile (65km) path of destruction.
It is said to have passed through several northern suburbs - including Mayflower where a witness described a twister half a mile wide crossing Interstate 40 on Sunday evening, the National Weather Service said.
Congressman Tim Griffin told Reuters news agency an "entire neighbourhood of 50 homes or so" in Faulkner County had been destroyed, with many "completely gone except the foundation".
Many homes and businesses, including a new secondary school worth $14m (£8.3m), were left in ruins in Vilonia after the storm.
"There's just really nothing there anymore. We're probably going to have to start all over again," said Vilonia schools chief Frank Mitchell after inspecting the wreckage of the school.
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