Tornadoes ripped through an East Texas county on Saturday evening, killing at least four people and injured dozens others.
Seven others were killed as a result of high winds, falling trees and floods in neighbouring states.
Three tornadoes were confirmed by the US National Weather Service in Canton, a city about 60 miles east of Dallas in Van Zandt County.
The winds flipped over cars, snapped trees, destroyed houses and left roads strewn with debris and fallen power lines, according to photographs and video published by the Dallas Morning News.
"We have at least four fatalities," Canton Mayor Lou Ann Everett said at a news conference on Sunday, adding that number could rise.
"The damage was extensive in the affected area." Forty nine people had been treated for injuries, she said.
Texas Governor Greg Abbott deployed a search and rescue team to the area.
The storms caused floods in neighbouring states, killing a 72-year-old woman in southwestern Missouri who was washed away in her car.
A 2-year-old girl in Tennessee died after being struck by a heavy, metal soccer goal post that was blown over by high winds.
In Arkansas, a woman was killed when a tree fell on her mobile home in DeWitt, and a 10-year-old girl was killed after flood waters swept her away in Springdale.
A volunteer fire department chief was killed while working during storms in north-central Arkansas, state police said.
Cove Creek/Pearson Fire Chief Doug Decker died after being struck by a vehicle while checking water levels on Highway 25 near Quitman, Trooper Liz Chapman said.
In Mississippi, a person was killed after a tree fell on their house in Durant, ABC reported.
The governors of Missouri and Oklahoma declared states of emergency due to flooding.
In the St. Louis area, severe thunderstorms were forecast through Sunday.
Flash floods and strong thunderstorms were forecast for parts of Louisiana, Mississippi and Alabama, the National Weather Service said.