A wave of tornado-spawning storms swept the US South, splintering buildings across hard-hit Alabama and killing 72 people in four states.
At least 58 people died in Alabama alone, including 15 or more when a massive tornado devastated Tuscaloosa.
Eleven deaths were reported in Mississippi, two in Georgia and one in Tennessee.
States of emergency were declared in Alabama, Arkansas, Kentucky, Mississippi, Missouri, Tennessee and Oklahoma, and governors called out the National Guard to help with rescue and clean-up operations.
The National Weather Service had preliminary reports of more than 110 tornado touchdowns in 24 hours.
Especially hard-hit was the southern state of Alabama, hit by two lines of storms, one in the morning and one in the evening.
An evening tornado tore through the city of Tuscaloosa and the city “has been majorly impacted,” Yasamie August with the Alabama Emergency Management Agency told AFP.
“We’re still trying to assess how bad the damage is,” said August, adding that the local EMA office was among the buildings struck.
News footage showed paramedics lifting a child out of a flattened Tuscaloosa home, with many neighbouring buildings in the city of more than 83,000 also reduced to rubble. A hospital there said its emergency room had admitted at least 100 people.
Tuscaloosa Mayor Walter Maddox told CNN that the tornado “obliterated blocks and blocks” of his city.
“My wife and I were watching the weather coverage until the monster was right up on us,” Tuscaloosa resident Will Nevin told The Birmingham News, the local newspaper.
“Then it was the mad dash to the bathroom where the lights flickered, pulsed and finally gave up. Even on the ground floor of our two-story apartment building, we could hear the wildness above us.”
A tornado also struck the city of Birmingham and officials were still assessing the damage, August said.
“This has been a very serious and deadly event that’s affected our state, and it’s not over yet,” Alabama Governor Robert Brantley told reporters after the second string of storms.
Some 74 tornadoes were reported on Wednesday and there have been hundreds of reports of wind and hail damage, the national weather service said.
The NWS issued a rare “high-risk” warning of tornadoes, hail, flash flooding and dangerous lightning for parts of Alabama, Georgia, Mississippi and warned that severe weather could also strike 21 states from the Great Lakes down to the Gulf Coast and across to the Atlantic.
The NWS said that a tornado watch will remain in effect until 0500 GMT Thursday for southeastern Alabama.
Storm victims across the region were trapped in homes, trailers and cars by falling trees. Hail the size of golf balls cracked windows.
Roads were washed out or rendered impassable by fallen trees and power lines across the region. Homes, schools and businesses were flattened, flooded and set on fire by lightning.
“Our citizens have endured days of consecutive severe storms and flooding,” said Kentucky Governor Steve Beshear.
“We stand ready to assist any community in need, and we urge Kentuckians to remain on alert until this storm system finally passes.”
Hundreds of people were evacuated from their homes in Missouri after levees failed to hold back swollen rivers.
“I’m just glad my family is safe,” said Chris Pigg, who spent the night at a shelter with his wife and daughter and wasn’t sure if he’d have a home to return to after the Black River breached the levee in Poplar Bluff, Missouri.
The skies are not expected to clear until late Thursday or Friday.
There will be little time to mop up, as another major storm system is forecast to bring heavy rain and high winds on Saturday.
The storm comes after a wet spring and a winter of heavy snowfall, which means the ground is saturated and rivers were already quite high.
Officials were considering deliberately destroying levees in some areas to ease pressure on swollen rivers, some of which are so high that barges have become trapped under bridges.
As much as 18 inches (45 centimetres) of rain had fallen from Saturday through Tuesday night in some areas.
“It’s producing major to record flooding in a lot of those river basins,” said Jim Keeney, deputy chief of the weather service’s central region.
In Arkansas, flash floods and a series of tornado strikes have killed at least 11 people.
“There’s trees down, the power’s down, you’ve got homes that are damaged… as well as a lot of flooding going on,” Renee Preslar, a spokeswoman for the Arkansas department of emergency management, told AFP.
Officials across the country begged people to use common sense and not to try to drive or walk through the floodwaters.
“Some people are still trying to drive around barricades with their cars. Please don’t do that,” Louisville, Kentucky Mayor Greg Fisher said at a press conference.
“If it’s moving fast you can go with it.”