Rescue teams in Mississippi searched for survivors after tornadoes tore through the southern US state, killing at least 10 people, injuring nearly two dozen and destroying homes.
The toll was expected to mount as rescuers cleared the debris after the storms tore roofs off buildings, reduced homes to rubble, overturned vehicles, downed power lines and toppled trees that blocked roads.
Mississippi Governor Haley Barbour declared a state of emergency in 17 counties devastated by the storms and twisters, and called on the National Guard to help local officials in their emergency response.
American Red Cross workers have also been dispatched to affected areas.
Downed trees and power lines damaged 30 homes and closed two roads in Warren County alone.
Strong tornadoes meanwhile developed in neighbouring states.
Four victims were flown by helicopter from Yazoo City to the University of Mississippi Medical Center in Jackson, while the American Medical Response service transported 17 others by ambulance to area hospitals, said Red Cross spokesman Jim Pollard.
Emergency response teams set up two shelters for storm victims in Vicksburg and Yazoo City.
The tornado that blasted through Yazoo City Saturday was nearly 1.6km wide with winds at about 241km/h, according to meteorologists.
"It reminds me of (Hurricane) Katrina," Mr Barbour told local reporters in his home town Yazoo City.
The storms cut through a large swath of the country, with areas in Mississippi, Alabama, Tennessee, Kentucky, Georgia, Ohio, Illinois and Indiana placed under tornado watches and warnings as severe thunderstorms swept over the region.
The National Weather Service Storm Prediction Center "is forecasting the development of a few strong, long-track tornadoes over parts of the mid-south, central Gulf Coast states and Tennessee Valley.
It said strong storms would continue to strengthen as they moved toward the northeast, possibly unleashing severe thunderstorms early on Sunday, including "strong and long-lived" tornadoes producing "extensive damaging winds," large hail and severe thunderstorms.