At least 20 tornadoes hit southern United States, damage houses and power lines

At least 20 tornadoes hit the southern United States on Wednesday, cutting power and damaging homes, the National Weather Service said, urging residents to seek shelter.

Tornadoes struck the states of Mississippi, Louisiana and Alabama, damaging roads, downing trees and blowing roofs off houses, NWS said.

NWS described one tornado ripping through Alabama as "life-threatening" in an advisory issued at 0845 GMT.

"Tornadoes are extremely difficult to see and confirm at night. Do not wait to see or hear the tornado. TAKE COVER NOW!"

Parts of Alabama and Mississippi were under a tornado watch until 6:00 am (1200 GMT) Wednesday, NWS said.

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Hail ranging from "quarter size" to "baseball size" was forecast for affected counties in Alabama, it said.

In the first week of November, a series of powerful tornadoes battered the southern US states of Oklahoma, Texas and Arkansas, leaving one person dead.

Tornadoes are a frequent and often devastating weather phenomenon in the United States, with the Great Plains states of Kansas, Oklahoma and Texas the hardest hit.

In December last year, dozens of devastating tornadoes ripped through five US states overnight, leaving at least 79 people dead in Kentucky -- with fatalities also recorded in Tennessee, Arkansas, Missouri and Illinois.