Bystanders were warned not to smoke because of leaking gas while police on Saturday patrolled to ensure there was no looting in Gallatin, Tennessee.
Emergency officials implemented a dusk-to-dawn curfew for the worst-hit areas, and National Guard soldiers were brought in to patrol.
Last weekend, thunderstorms spinning out dozens of tornadoes killed 24 people in western Tennessee and four others in Missouri and Illinois.
Later on Friday and into Saturday, another line of severe thunderstorms rolled through Alabama, Georgia and South Carolina.
In Tennessee alone, tornadoes were spotted in about 10 counties on Friday, weather officials said.
The worst damage appeared to be in Gallatin and other suburbs northeast of Nashville.
Nine people were killed in Sumner County and three were killed in Warren County, about 105km southeast of Nashville, Randy Harris, the state Emergency Management Agency spokesman, said on Saturday.
Hospitals admitted at least 60 people with storm-related injuries.
Phil Bredesen, Tennessee's governor, toured the destruction on Saturday.
Nashville Electrical Service reported hundreds of electrical lines down and power outages affecting up to 16,000 customers.
Tornadoes were reported in 10
Tennessee counties on Friday
The number of customers blacked out was down to 1,100 by Saturday, but some people might have to wait a week for their service to be restored, the power company said.
The National Weather Service said four tornadoes swept through Georgia. The twisters destroyed businesses, homes and at least one church in the Atlanta area and knocked out power to tens of thousands, authorities said. No deaths or serious injuries were reported.
Several people were injured in Alabama, two by falling trees, but no deaths were reported, officials said on Saturday.
A store was destroyed in Ohatchee, near Anniston, and homes and apartments were damaged in the Birmingham area. Storms also pounded southern West Virginia, blacking out more than 16,000 customers, utilities said.
One person was injured in South Carolina as three tornadoes touched down around Charleston, the weather service said.
One tornado touched down at the Family Circle Cup tennis facility at Daniel Island, damaging banners and a score board and interrupting the tournament's qualifying rounds.
The weather service confirmed that a tornado touched down in southern Kentucky on Friday, destroying or damaging dozens of mobile homes. Four people suffered minor injuries.