(CNN) -- The death toll from a wave of violent storms that swept across the South skyrocketed to 173 after county-by-county reviews by Alabama and Mississippi emergency management agencies Thursday turned up scores of additional fatalities that more than doubled the total, officials said.
The vast majority of fatalities occurred in Alabama, where at least 128 people perished, Jennifer Ardis, a spokeswoman for Gov. Robert Bentley, told CNN Thursday. A breakdown provided by Ardis showed that violent weather claimed lives in 16 Alabama counties. The hardest hit was DeKalb County, Alabama, where 30 people perished in the storms.
Before dawn Thursday, Mississippi emergency management officials also added 14 previously unreported fatalities to the count, increasing the death toll in that state to 32, officials said.
The storms left fatalities in five southern states, including Arkansas, Georgia and Tennessee. They leveled neighborhoods and left hundreds of thousands of customers without power throughout the region.
"This could be one of the most devastating tornado outbreaks in the nation's history by the time it's over," CNN Meteorologist Sean Morris said.
Long before the death toll mushroomed, governors in Alabama, Mississippi and Georgia had declared states of emergency within their borders.
President Barack Obama announced late Wednesday he had approved Bentley's request for emergency federal assistance, including search and rescue support.
"While we may not know the extent of the damage for days, we will continue to monitor these severe storms across the country and stand ready to continue to help the people of Alabama and all citizens affected by these storms," Obama said in a statement.
At least one strong tornado swept through Tuscaloosa, Alabama, leaving dozens of roads impassable and destroying hundreds of homes and businesses.
"It literally obliterated blocks and blocks of the city," Mayor Walter Maddox said, describing Tuscaloosa's infrastructure as "decimated."
Witnesses also reported tornado touchdowns in Birmingham, Alabama.
"It looked like it was probably a mile wide," Birmingham Mayor William Bell said.
The northwest corner of the city was particularly devastated, he said, with hundreds injured and many others missing.
Red Cross spokesman Chris Osborne said the number of ambulances on the street in Birmingham, "is just like taxicabs in New York."
"It's just back and forth to area hospitals," Osborne said. "It's really just an incredible sight to see."
Osborne said Pratt City and Pleasant Grove were among the hardest hit areas.
"It's just bare land, debris everywhere," Cierra Brown, of Jefferson County, Alabama, told CNN affiliate WBMA about her devastated neighborhood. "There's no house."
"My bathroom is across the street," Talesha Oliver told WBMA.
Henry Nguyen told CNN early Thursday he was working at his father's convenience store on the edge of Pratt City when he saw a twister angling for the front door. He ducked. When he stood up, Nguyen said he saw that the tornado had missed the storefront by 50 yards.