The violent storms included what may be the worst ever twister to hit Alabama. It killed 15 people as it ripped through the university city of Tuscaloosa, crushing houses, picking up cars and uprooting trees by the hundreds.

Deaths occurred in Arkansas and Mississippi, where 11 were killed in each state, and also in Louisiana, Georgia and Tennessee. But Alabama appeared the hardest hit.

The Alabama Emergency Management Agency (AEMA) confirmed 45 storm-related deaths in nine different counties across the northern and central parts of the state, with at least nine people also injured, authorities said.

Authorities there and in Mississippi said they expect the toll to rise as emergency workers attempt rescues and recovery in the storm's wake.

Tornadoes are a regular feature of life in the South and Midwest but rarely are they so devastating.

President Barack Obama declared a state of emergency for Alabama and ordered federal aid for the stricken state.

Governors in Alabama, Arkansas, Mississippi and Tennessee also declared a state of emergency.

Alabama Governor Robert Bentley mobilized around 1,400 National Guardsmen and said they would be on the ground early on Thursday to help with search and rescue, logistics and debris removal, a statement from his office said.