Tornadoes and storms in the south-eastern United States have killed at least 250 people, officials say.
In Alabama, the worst-hit state, more than 162 have died in recent days - including 36 killed by a tornado that devastated the city of Tuscaloosa.
Deaths and widespread devastation are also reported in Tennessee, Mississippi, Georgia and Virginia.
A state of emergency has been declared in seven states, and federal aid money is being sent to Alabama.
US President Barack Obama said the damage was "nothing short of catastrophic", and pledged the government would do everything it could to help the communities affected by the storms.
Mr Obama said he would visit Alabama on Friday to see the damage for himself.
The US National Weather Service has reports of nearly 300 tornadoes since the storms began on Friday, more than 150 of them on Wednesday alone.
In Alabama, as many as one million people were without power on Thursday morning, as emergency workers and 2,000 soldiers scoured the wreckage for survivors.
Governor Robert Bentley said he expected the death toll to rise as more bodies were discovered.
He said Alabama residents are accustomed to tornados and had taken precautions, but "in highly populated areas, it just makes it very difficult to move everyone out when a tornado comes through that's a mile wide".
Tuscaloosa - home to more than 83,000 residents, and to the University of Alabama - was hit on Wednesday evening.
"I don't know if I've ever seen in my life anything as destructive," Mayor Walter Maddox said.
Northern and central parts of the state bore the brunt of the latest storms. Eleven people died in Jefferson County, home to Birmingham, Alabama's largest city.
A shop-owner in Birmingham told AFP news agency that 30 homes near his store had been destroyed by a tornado.
On Wednesday night, President Obama declared a state of emergency in Alabama, allowing federal authorities to help co-ordinate disaster relief and to provide aid.
Mr Obama said he had spoken to Governor Bentley and approved his request for emergency assistance, including search-and-rescue teams.
More rain coming
States of emergency were also declared in Arkansas, Kentucky, Mississippi, Missouri, Tennessee and Oklahoma, following the latest storms and tornadoes.
The storms forced the Tennessee Valley Authority - which provides electricity to nine million people in seven states - to close three nuclear reactors at a power plant in Alabama. Hundreds of thousands of homes have lost power as a result.
"We have never experienced such a major weather event in our history," the Tennessee Valley Authority said in a statement.
In Tennessee, storms and subsequent flooding on Wednesday killed at least 33 people, the state emergency management agency said on Thursday.
Mississippi reported 33 deaths on Tuesday and Wednesday - including that of a police officer who shielded his nine-year-old daughter from a falling tree while on a camping holiday. The girl escaped unhurt.
At least 13 people have been killed in Georgia and eight in Virginia.
The current storm system is forecast to hit North and South Carolina before making its way further north-east.
Storms have hit states across the southern US for weeks, and another major storm system is forecast to bring heavy rain in the coming days.