More storms are forecast in what is often called "Tornado Alley" in the US central and southern plains. Casualties appeared limited because many of the twisters hit sparsely populated areas, and during daylight hours or evening when people were still awake.
However, in Oklahoma, a twister struck the north-west city of Woodward early on Sunday after lightning apparently disabled its storm warning system, mayor Roscoe Hill said.
Two children died at the Hide A Way mobile home park on the west side of Woodward, population 12,000, while two adults were killed in a small community just outside the town, Hill said. Details of the fifth death were not immediately known, according to Keli Cain, spokeswoman for Oklahoma Emergency Management.
"This thing took us by surprise," Hill said, adding that storm sirens had not sounded. "It's kind of overwhelming."
Hill said he was told the tornado hit the west and north sides of the city, badly damaging an apartment complex where residents were trapped and awaiting rescue.
A tornado that struck Woodward in April 1947 still ranks as the deadliest in Oklahoma history, with 116 people killed, according to the National Weather Service.
In Kansas, a twister churned through parts of Wichita, the state's second-largest city. Brandon Redmond, a meteorologist, said the twister passed over his vehicle and lifted it off the ground in an industrial area.
"The tornado literally formed over our vehicle," he told Reuters. "I've never been that scared in my life … We had power flashes all around us and debris circulating all around the vehicle, sheet metal, parts of a roof, plywood."
Redmond said there was significant damage in vicinity but there were no immediate reports of injuries.
Residents in the affected states – Oklahoma, Kansas, Nebraska and Iowa – and in an area stretching from Minnesota to Texas hunkered down for more severe weather. Forecasters said the worst conditions were expected in Oklahoma, Nebraska and Kansas, while other areas could see baseball-sized hailstones and strong winds.
"Conditions will remain very favourable … for very strong and potentially long-lived tornadoes," the National Weather Service said.
It warned that night-time tornadoes could be particularly dangerous because they are usually fast-moving and often obscured by rain and darkness.
The tornado season started early this year, with twisters already blamed for 62 deaths in 2012 in the midwest and south, raising concerns that this year would be a repeat of 2011, the deadliest tornado year in nearly a century.
Some 550 people died in tornadoes last year, including 316 killed in an April outbreak in five southern states, and 161 people in Joplin, Missouri, the following month.
On Sunday morning, the National Weather Service website said it had received preliminary reports of 121 tornadoes across four states over the previous 24 hours.
