A tornado caused heavy property damage in Alabama, with other southern US states at risk from the swarm.
Serious structural damage was reported in Birmingham, Alabama's largest city, in the Lee Branch, Eagle Point, Greystone and Dunavant sections. Several homes in Eagle Point were destroyed.
Alerts are in place in Mississippi and Tennessee, with Huntsville, Memphis, Nashville and other cities under threat.
Twitter users posted a video of twisters near Birmingham.
CNN meteorologist Derek Van Dam heard warning sirens wailing across the city.
Photos show damage from the storm across Alabama.