Tornadoes killed at least five people and wreaked massive damage late Friday in Oklahoma near the battered city of Moore, still recovering from a monster twister less than two weeks ago.
The Oklahoma Medical Examiner's office confirmed the death toll, but said it had no further details on the victims.
But the Oklahoma Department of Emergency Management confirmed a mother and child were killed as they traveled in their car on Interstate 40 near Oklahoma City.
At least 12 people were injured in the storm, with that number expected to rise, the agency told AFP.
Several people were hurt in crashes and overturned vehicles, including semi-trailers and trailers, Oklahoma Highway Patrol spokeswoman Betsy Randolph told NBC television affiliate KFOR.
"It is a heartbreaking situation," Randolph said.
While storms continued to pass through the state, Department of Emergency Management spokeswoman Keli Cain said, "they're much less severe." However, "there's still a lot of flooding in areas," she said, warning residents to stay off roads and avoid high water.
Weather forecasters lifted an emergency for parts of the tornado-prone midwestern state, though flash flood warnings, severe thunderstorm warnings and tornado watches remained in place.
National Weather Service's Storm Prediction Center in Norman, Oklahoma warned the severe weather was shifting eastward Saturday, with the Ohio and Mississippi River Valleys set to receive the bulk of the storms.
"Some of the rainfall is expected to be quite heavy and a threat of severe weather will continue," the weather center said on its website.
In Missouri, which borders the Mississippi River, Governor Jay Nixon declared a state of emergency Friday night, urging "Missourians to closely monitor weather conditions, so they can take shelter or move to higher ground if needed."
Reports said five twisters had struck the area around Oklahoma City, with winds of up to 90 miles (145 kilometers) per hour, accompanied by very large hail.
Flash floods also hit the area, the Tulsa World newspaper said.
One large tornado touched down west of Oklahoma City, the state's biggest city, news reports said.
Oklahoma Governor Mary Fallin said earlier she was "heartsick" that Moore, struck by a powerful tornado on May 20, could be hit once again.
"I'm very concerned right now," Fallin told CNN.
"I'm here across from the capitol and it is hailing and blowing and raining extremely hard. Our tornado sirens have gone off several times."
KFOR television, meanwhile, reported extensive damage around the cities of El Reno and Yukon. More than 170,000 people were said to have lost power in the Oklahoma City metro area.
Officials for Oklahoma City's Will Rogers World Airport said on Twitter that passengers had been evacuated to an underground tunnel and that flights in and out had been suspended.
The Moore tornado affected a total of 33,000 people, with winds above 200 miles per hour.
With an average of 1,200 tornadoes per year, the United States is the most hurricane-prone country in the world. They are particularly prominent in the Great Plains states of Kansas, Oklahoma and Texas, as well as in Florida.