A mile-wide tornado has torn through six southern US states, killing at least 297 people, with authorities warning of more to come.
Widespread devastation has been reported in Alabama, Tennessee, Mississippi, Georgia and Virginia.
President Barack Obama is to visit communities in Alabama ravaged by the worst storms since 1974. He will visit Tuscaloosa, Alabama, and has pledged government support for storm-hit communities, with federal aid money being sent to Alabama.
A state of emergency has been declared in seven states, and federal aid money is being sent to Alabama.
Speaking at a news conference at the White House, Mr Obama said: “The loss of life has been heartbreaking, especially in Alabama.
“In a matter of hours, these deadly tornadoes, some of the worst we have seen in decades, took mothers and fathers, sons and daughters, friends and neighbours, even entire communities.
“In many places the damage to homes and businesses is nothing short of catastrophic,” he said.
“I want every American who has been affected by this disaster to know that the federal government will do everything we can to help you recover, and we will stand with you as you rebuild,” he said.
The national weather service said the storms were the most ferocious some of their forecasters had ever seen; the deadliest since tornadoes in 1974 killed 315 people.
As well as the 210 people killed in Alabama, Mississippi reported 33 dead, Tennessee 33, and Georgia 15, with five in Virginia and one in Kentucky.
The tornadoes also forced a temporary shutdown at a nuclear plant in Alabama, but the nuclear regulatory commission said there was no danger.
US weather forecasters had been warning for days of a powerful storm coming up out of the south-east. Schools were shut and many people took a day off work. However, those precautions were overwhelmed by the sheer force of the storm system.
The tornadoes took up a far wider stretch of ground than typical twisters, and stayed on the ground much longer. A number had wind speeds of more than 200mph, the weather service said.