The Lee County Emergency Management Agency said the worst of the damage was near the town of Beauregard.
One man living in the area said the tornado's advance sounded like a "freight train coming."
After nightfall Sunday, the rain had stopped and pieces of metal debris and tree branches littered roadways in Beauregard. Two sheriff's vehicles blocked reporters and others from reaching the worst-hit area. Power appeared to be out in many places.
Search and rescue efforts were due to resume in full force at daybreak, Jones said.
Jonathan Hickman, who lives in Beauregard, told NBC News he and his mother, Liz, saw the tornado pass through and barely got out alive.
They said many homes appeared to have been destroyed.
"Everything got wiped. It's been leveled," Hickman said in a Facebook Live posted while walking around the neighborhood. "Prayers for the people who lost everything they have ever had."
Liz Hickman was huddled with her grandchildren when the tornado hit.
"We’re doing OK. I guess best to be expected."
She told her grandchildren: “Y’all just hold tight. We’re here. We were just holding on so tight.”
Radar and video evidence showed what looked like a large tornado crossing the area near Beauregard shortly after 2 p.m. Sunday, said meteorologist Meredith Wyatt with the Birmingham office of the National Weather Service.
"It appears it stayed on the ground for at least a mile and maybe longer," Jones told the Associated Press.
The NWS' preliminary assessment of the tornado put it at F3 on the Fujita scale. F3 storms typically are gauged at wind speeds of between 158-206 mph.
The tornado is the deadliest since 2013, when an F5 tornado killed 24 people in Moore, Oklahoma.
"This is a day of destruction for Lee County," county Coroner Bill Harris told NBC affiliate WSFA of Montgomery. "We've never had a mass fatality situation, that I can remember, like this in my lifetime."
"I wouldn't wish this on anybody," Jeremy Daniel, a Lee County resident, said. "I've seen earthquakes, and we had a fire last year -- me and my family. This just came on so quick and changed so many lives."
The NWS said the tornadoes touched down amid a severe weather outbreak across the Southeast.
"It's a widespread storm," Brian Hastings, director of the Alabama Management Agency, said in an interview with WSFA.
"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" in several neighboring counties, Hastings said.