Storms and tornadoes that hit the southeastern US over the weekend have left at least 47 people dead in six states.
The National Weather Service said the worst was still to come, NBC reported.
Emergency workers were looking for victims in the hardest hit areas of North Carolina, which was slammed by 62 tornadoes in what is being considered the worst spring storm in two decades.
The storms in North Carolina caused 24 deaths, 14 of them in one county. Authorities warned that the number of fatalities could increase.
North Carolina Governor Beverly Perdue declared a state of emergency and said it was the worst storm since March 1984, when a similar disaster killed 57 people - 42 in North Carolina - and injured hundreds more.
"Our thoughts and prayers are with everyone in North Carolina who experienced this horrible day," Perdue said.
Thousands of people were without power after the storms.
Tornadoes and heavy rain killed two people in Oklahoma, nine in Arkansas, seven in Alabama, one in Mississippi and four in Virginia.
The Milk river in eastern Montana, James and Big Sioux rivers in South Dakota, Minnesota river and the upper portion of the Mississippi river basin from Minneapolis to St. Louis have been put on alert.
Officials recommended that the public monitor the news and be prepared in case they have to seek shelter or be evacuated.