A deadly tornado smashed through the small US city of Hattiesburg, Mississippi, early on Saturday morning.
There was widespread damage to buildings. Power lines were brought down and a number of cars were flipped over.
Four people were killed by the twister, and it is possible that there are others still trapped beneath the storm debris, Reuters news agency reported.
According to preliminary reports from the National Weather Service, the tornado was an EF-3 on the Enhanced Fujita scale, meaning severe damage and winds of 220 to 265 kilometres an hour.
It ripped a path of destruction 40km long and 1km wide across parts of four counties. Mayor Johnny DuPree said that of the four killed, three died in their trailer homes.
Governor Phil Bryant issued an emergency declaration and added that he would be consulting with White House officials soon. Hattiesburg, with a population of 45,000, is the largest city in Forrest County.
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At least 28 people suffered injuries, none thought to be life-threatening.
At the height of the emergency, more than 16,000 homes and businesses across Mississippi were without electricity, according to emergency management officials.
Tornadoes were also spotted in neighbouring Alabama.
The threat of violent weather remains. Warnings of severe thunderstorms accompanied by a risk of damaging winds, large hail and tornadoes have been issued across parts of the southeast throughout Sunday.