A woman has died and thousands of homes and businesses are without power after a tornado hit New Orleans overnight.
Among the worst-hit areas were the Lower 9th Ward and parts of St Bernard Parish, where the fatality occurred. Several other people have been injured, but there are as yet no further details.
Both areas were badly damaged by Hurricane Katrina in 2005.
Some 17,000 properties were without power in Louisiana, almost 26,000 in Texas, and more than 11,000 in Mississippi, according to website poweroutage.us.
New Orleans resident Reggie Ford told The Associated Press: “I see down powerlines, a church is completely destroyed, three businesses are completely destroyed.
“There are eight blocks of houses missing their roofs.”
In the suburb of Arabi, some houses were destroyed and debris was hanging from wires and trees.
Arabi resident Michelle Malasovich said she had been texting family when the lights in her home started flickering.
She said she had noticed that it had seemed very windy, adding: “It just kept getting louder and louder.
“Our neighbour’s house is in the middle of the street right now.”
Much of southern Louisiana is still recovering from Hurricane Ida – a Category 4 storm that hit the area last August.
Guy McGinnis, president of St Bernard Parish, told WWL-TV that the parish had seen “widespread damage” overnight.
Search and rescue teams were responding to at least two calls from people who said they were trapped in their homes, he said.
Other tornadoes spawned by the same storm system hit parts of Texas and Oklahoma, killing an elderly woman about 60 miles north of Dallas, injuring more than 20 people, and causing widespread damage.
In Jacksboro, about 60 miles northwest of Fort Worth, parts of the local high school lost a roof and walls.
Texas governor Greg Abbott announced a disaster declaration for 16 of the worst-hit counties.
The storms are expected to get worse during Wednesday and, as temperatures rise, the risk of more tornadoes will increase.
Forecasters have said that much of Mississippi, southern and eastern Louisiana, and western Alabama could see winds of up to 75mph.