At least four people have died, including a four-month-old baby, and scores have been injured in Oklahoma after dozens of twisters swept the US Southern Plains, while weather alerts put more than seven million Americans under tornado warnings.

Storm warnings for high winds, heavy rain and hail also were issued by the National Weather Service on Sunday (local time) for more than 47 million people stretching from East Texas all the way up through Illinois and Wisconsin.

The unsettled weather was forecast to continue across the mid-section of the US into Monday, the weather service said.

On Sunday, Oklahoma Governor Kevin Stitt declared a “disaster emergency” for the state, freeing up more money for first responders and recovery operations. Stitt said in a video message posted on social media that he planned to tour the storm damage on Sunday and asked for prayers for those suffering.

Nearly 30,000 people remain without electricity after tornadoes began late on Saturday. The destruction was extensive in Sulphur, a town of about 5000 people, where many downtown buildings were reduced to rubble and roofs were sheared off houses across a 15-block radius.

The NWS reported 38 possible twisters hit the area and that the worst of the storms rolled through Central Oklahoma on Saturday into early Sunday morning, spreading into north-west Texas, western Missouri and Kansas.

The deaths were reported by the Oklahoma Department of Emergency Management, which also reported more than 30 injuries, and dozens of homes destroyed of damaged.

The agency cautioned that the extent of the destruction was still being assessed, amid a mess of downed trees and power lines and debris-strewn roads.

-AAP