Eight people are reported to have been killed in Texas after new tornadoes, raising the death toll to 26 in a week of storms in several US states.
Local media said five people in cars were probably blown off a motorway in Garland, near Dallas. Another three bodies were found in nearby towns.
Officials also say Texas and Oklahoma could suffer a “historic blizzard”, bringing up to 16in (41cm) of snow.
The storms across the South have been unusually powerful for winter.
Reports from Texas said churches were destroyed, cars mangled and trees toppled across a wide area.
Garland police believed that “winds from a tornado that passed through” the town late on Saturday were the cause of car accidents, Melinda Urbina from the Dallas County Sheriff’s office told the BBC.
At least five people were reported dead in the cars.
Ms Urbina said the winds “tossed the cars around”, and the vehicles were later found below Interstate 30, about 15 miles (24km) north-east of Dallas.
She also urged local residents to stay off the roads.
Police officers in Garland are now trying to determine whether they were any other casualties at the crash site.
“We’re dealing with darkness out here,” police spokesman Mike Hatfield was quoted as saying by the Dallas News website.
“All of the street lights and highway lights are out,” he added.
Two people were found dead at a petrol station in Copeville, and a third was killed in Blue Ridge, reports in local media said.
The storms have damaged a number of houses in the area, including churches.
Several power lines were down and there were reports of burst gas lines.
The National Weather Service confirmed that tornadoes had touched down near Dallas and several other towns in northern Texas.
The deaths in Dallas come as much of the south-central region of the US is hit by severe weather.
While extreme weather in the US before Christmas is not unknown, meteorologists say that unseasonably high temperatures in some areas contributed to the severity of the storms.
One year ago a tornado hit south-eastern Mississippi, killing five people and injuring dozens more.
A storm on Christmas Day in 2012 which included several tornadoes damaged homes from Texas to Alabama.
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Source: BBC