Storms that spawned deadly tornadoes have flattened buildings and knocked out powerlines across the central and southern United States, raising the death toll from severe weather to at least 56 people.
Authorities said on Wednesday (local time) that 45 people had been killed in Alabama alone.
US president Barack Obama says emergency aid assets will be forthcoming for the south-eastern state.
“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,” he said.
Earlier, states of emergency were declared in Alabama, Arkansas, Kentucky and Mississippi, where officials said 11 people were killed.
Governors have 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 between 12:00pm Wednesday) and 2:00am Thursday.
Especially hard-hit was the southern state of Alabama, hit by two lines of storms: one in the morning and one in the evening.