The death toll from tornadoes that caused massive devastation in the southern United States has risen above 350 as President Barack Obama expressed his grief.

'We're going to make sure that you're not forgotten,' Mr Obama said on a visit to the city of Tuscaloosa yesterday after meeting victims whose homes were demolished.

'I've never seen devastation like this. It is heartbreaking.'

In Alabama alone, Wednesday's deadly storms claimed 255 lives, the state Emergency Management Agency reported.

In addition, 1,730 people in the state were either hospitalised or injured, while 20 others were listed as missing.

At least 101 more deaths were reported across Mississippi, Tennessee, Arkansas, Georgia, Virginia and Louisiana.

The toll made the storms the third-deadliest tornado tragedy in US history. The worst tornado outbreak in the US in March 1925 left 747 people dead.

Mr Obama and First Lady Michelle Obama visited Tuscaloosa to see the destruction firsthand.

They also met Alabama Governor Robert Bentley and shook hands and hugged distraught residents.

Mr Obama pledged help for Tuscaloosa and dozens of other cities and towns across the US south struck by the worst US natural disaster since Hurricane Katrina in 2005.

'We can't bring those who have been lost back, they are alongside God at this point,' Mr Obama said, but he pledged 'maximum federal help' to cope with property damage and recovery costs.