Tornadoes have struck the US states of Nebraska, Kansas and Oklahoma following a warning by forecasters to residents across the US Midwest about "life-threatening" weather.
The National Weather Service said in an advisory on Saturday afternoon that severe storms were possible from Texas in the south to Iowa, South Dakota and Minnesota in the north.
"Conditions will remain favourable for strong to violent and possibly long-lived tornadoes into the overnight hours," it said.
Tornadoes during the overnight hours can be particularly dangerous because people are often sleeping and don’t hear the warnings, plus it’s impossible to see them coming in the dark.
Local television station KOCO in Oklahoma City reported that the death toll reached four on early Sunday morning.
Citing the local medical examiner, KOCO reported that all four deaths occurred in Woodward just after midnight, and that two of the deaths were children.
KOCO also said that there the twister in Woodward caused "significant structural damage and several injuries".
In the Iowa town of Creston, the Greater Regional Medical Center hospital was damaged by a possible tornado, said a woman who answered the phone there but declined to give her name.
A dispatcher for the Union County sheriff's office said he was unable to release any information about damage to the hospital or buildings.
In northwestern Oklahoma, a tornado touched down for less than a minute in the afternoon, according to Rick Smith, a National Weather Service meteorologist in the town of Norman.
The area was, however, hit by severe hail, breaking windows and damaging buildings.
Emergency operations
A tornado was also spotted in Mustang, a suburb of Oklahoma City, before dawn. Kristy Yager, a spokeswoman for Oklahoma City, said that trees, power lines and fences were reported to have collapsed in the area.
"We'll have storms all night long in Oklahoma," Smith said. The pair of Oklahoma twisters hit a day after a tornado sliced across Norman on Friday afternoon.
The Federal Emergency Management Agency said the worst conditions were expected to hit late on Saturday between Oklahoma City and Salina, Kansas, while other areas could see baseball-sized hail and strong winds.
Oklahoma activated its emergency operations center as a precaution before the storms.
"We really want to make sure that the public is aware that this is a serious threat and make sure that people are prepared," said Keli Cain, a spokeswoman for the Oklahoma Department of Emergency Management.
The National Weather Service could not immediately confirm the storm was a tornado.
The US tornado season started early this year, with twisters already blamed for 57 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.
