Tornadoes have hit a large swathe of the US Midwest, killing at least five people in Oklahoma, officials say.
Twisters were also reported in Kansas, Iowa, and Nebraska as the storm front swept east.
The deaths occurred before dawn in the town of Woodward, Oklahoma, after warning sirens failed to sound, officials said.
A tornado there caused extensive damage, mainly in the west of the town. At least 29 people were injured.
The National Weather Service had forecast further tornadoes early Sunday morning, with concern they could strike as people slept.
Officials feared people would not hear warnings as they slept and said that it was more difficult for weather spotters to track the funnel clouds overnight.
Rescue teams searched rubble for people trapped or wounded in Woodward, where high winds damaged homes, uprooted trees and brought down electricity lines, the Associated Press reported.
A block of flats was also damaged by the twister, after residents were caught by surprise as the storm sirens had failed to sound, Reuters news agency quoted the local mayor as saying.
"This thing took us by surprise," Keli Cain, spokeswoman for Oklahoma Emergency Management, told the agency. "It's kind of overwhelming."
"They're still going door to door and in some cases there are piles of rubble and they are having to sift through the rubble," one of Ms Cain's deputies, Michelann Ooten, told AP.
In Iowa, a tornado destroyed large parts of the town of Thurman, on Saturday, but there were no major injuries, the NWS said.
"It lasted three to four minutes probably - what seemed like an eternity," one man from Thurman told the broadcaster ABC.
"The next thing I know, the house was shaking and I could feel it lifting and it was over that quick," another man said.
Another twister caused widespread power outages and other damage in the city of Wichita, Kansas, according to Associated Press.
The roof of a hospital in Creston, southwest of Des Moines, was damaged, but patients and staff were not hurt, AP reported.
Tornado experts had said that storms on Saturday could be a "life-threatening event".
US tornadoes have already killed at least 39 people in 2012.
An outbreak of deadly twisters hit the states of Indiana, Kentucky, Ohio, Georgia and Alabama in early March.
At the start of April the Dallas-Fort Worth area was badly hit, with hundreds of flights being disrupted but no-one injured or killed.