US president Barack Obama has promised federal aid to the tornado-ravaged south, where deadly tornadoes have killed at least 340 people and caused billions of dollars in damage.
Mr Obama toured damaged homes and met survivors on a visit to the worst-hit state, Alabama. It was one of seven southern US states mauled by recent tornadoes and storms which have caused insured losses of between $2 billion and $5 billion, according to estimates.
"We are going to do everything we can to help these communities rebuild," Mr Obama told reporters in Tuscaloosa, a university city in Alabama that was devastated by the tornadoes.
The destruction inflicted this week by the twisters, which flattened whole neighbourhoods, was the deadliest US natural catastrophe since Hurricane Katrina in 2005.
"I have never seen devastation like this. It is heartbreaking," said Mr Obama, accompanied by his wife Michelle and Alabama Governor Robert Bentley. "This is something I don't think anyone has seen before."
In Alabama, emergency officials again raised the death toll from the tornadoes in that state, to 238. Mr Bentley said 1,700 people were injured. At least 101 more deaths were reported across Mississippi, Tennessee, Arkansas, Georgia, Virginia and Louisiana.
FEMA Administrator Craig Fugate said it was feared the number of deaths would rise as states searched for many people unaccounted for. But the number of missing was not clear.
"We can't bring those who've been lost back. They're alongside God at this point ... but the property damage, which is obviously extensive, that's something we can do something about," Mr Obama said.
Mr Obama was eager to show that federal relief is on its way and that he is not taking the disaster lightly. His predecessor George W Bush was fiercely criticised for what was viewed as a slow response to Hurricane Katrina.
Flying into Tuscaloosa aboard Air Force One, Mr Obama and his family saw a wide brown scar of devastation several kilometres long and hundreds of metres wide.
The storms left up to 1 million homes in Alabama without power. Water and garbage collection services were also disrupted in some areas.
Alabama's Jefferson County, which is fighting to avoid what would be the largest municipal bankruptcy in US history, suffered damage and 19 dead but said the storms would have little direct impact on its struggling finances because federal grants were expected.
Reuters