Tornadoes and thunderstorms ravaged four states in the American South on Tuesday night, killing at least 20 people, injuring dozens and causing widespread damage, emergency services and local media said.

The violent storms swept across Arkansas, Tennessee, Kentucky and Mississippi, overturning trucks, trapping people, and smashing houses.

Two of the states hit by the tornadoes -- Arkansas and Tennessee -- were involved in "Super Tuesday" as a total of 24 states across the country held nominating contests ahead of November's presidential election.

Several candidates expressed condolences to the victims as they addressed supporters and there were media reports that at least four polling stations in western Tennessee were closed because of the storm.

In Arkansas, emergency services reported 11 dead after tornadoes hit as many as eight counties.

"It's a pretty rough night in the scope of it. I don't know if I can remember when we've had as many (tornado) warnings and

touchdowns," Arkansas Gov. Mike Beebe, said a telephone interview from an emergency operations centre in North Little Rock.

The governor's spokesman, Matt DeCample, said there was "no clue" as to how many were injured. "We're getting answers back in the multiples, but we're still looking for folks," he said.

In Kentucky, at least three people were killed at a mobile home park, the Louisville Courier-Journal newspaper reported.

Six more died in Tennessee, according to the Nashville Tennessean newspaper, and more than two dozen others were injured, some critically.

WIDESPREAD DAMAGE

Extensive damage in Tennessee included part of a shopping mall in Memphis and a dormitory at Union University in Jackson, where some students were trapped for a time but not seriously injured, according to the Web site of the Memphis Commercial Appeal. The newspaper quoted a National Weather Service spokesman as saying the Memphis area had been hit by a "pretty significant tornado."

CNN reported as many as 86 injuries and an unknown number of fatalities from the storm system, which swept through Arkansas before moving into Tennessee.

ABC affiliate WAPT in Jackson, Mississippi reported that a 50-foot (15-metre) wall had collapsed at the Sears store in the Hickory Ridge Mall in southeast Memphis and a building caught fire along State Line Road at Airways Boulevard.

Citing local officials, WAPT reported that an unknown number of people were trapped in a nearby industrial plant.

The Jackson Sun reported that a nursing home had been seriously damaged but the 114 residents were evacuated with no injuries reported.

The Nashville Tennessean newspaper, citing the Fayette County Sheriff's Department, said one man had been found dead north of Somerville, Tennessee.

The paper reported that the National Weather Service had recorded a half dozen tornadoes in Tennessee and northern Mississippi.

It also reported that 60 tractor-trailers had crashed on an interstate highway.