Emergency workers were at the scene pulling bodies and the injured out of the rubble of hundreds of homes damaged by the severe weather.
"The challenge is the sheer volume of the debris where all the homes were located," Sheriff Jay Jones told CNN. "It's the most I've seen that I can recall."
Tornado warnings were in force across Georgia and Alabama on Sunday evening.
Drones flying overhead equipped with heat-seeking devices had scanned the area for survivors but the dangerous conditions halted the search late on Sunday, Mr Jones said.
An intense ground search would resume Monday morning.
Mr Jones said said the twister travelled straight down a key local artery in Beauregard and that the path of damage and destruction appeared at least half a mile wide.
Several people in Lee County were taken to hospitals, “some of them with very serious injuries”, he added.
The National Weather Service confirmed late on Sunday that a tornado with at least an F3 rating caused the deadly destruction in Alabama.
Radar and video evidence showed what looked like a large tornado crossing the area near Beauregard shortly after 2pm, said meteorologist Meredith Wyatt with the Birmingham, Alabama, office of the National Weather Service.
Kay Ivey, Alabama's governor, warned residents on Twitter that more severe weather might be on the way.
She said the state was working to help families who had been impacted.
"Our hearts go out to those who lost their lives in the storms that hit Lee County today," Ms Ivey wrote.
"Praying for their families & everyone whose homes or businesses were affected."
The storm left more than 10,000 customers without power, the Birmingham News said, citing the utility Alabama Power.
Brian Hastings, director of the Alabama Management Agency, said: "We have historic flooding to the north and historic flooding on the Tombigbee [River], and now this storm system that just went through, and now we're getting reports of significant damage."
President Donald Trump tweeted late on Sunday night, urging Alabama residents to take care in the wake of the disaster.
“To the great people of Alabama and surrounding areas: Please be careful and safe. Tornadoes and storms were truly violent and more could be coming,” he wrote.
“To the families and friends of the victims, and to the injured, God bless you all!”
Additional reporting by agencies