Violent thunderstorms unleashed tornados and flash floods over southern United States over the weekend killing at least 20 people and forcing the Kentucky governor to declare a state of emergency.

Governor Steve Beshear said Monday four people died in weather-related accidents in his state and requested federal assistance to deal with storm damage.

"I urge individuals who encounter high waters to use extreme caution and avoid unnecessary contact with flood waters if at all possible," he said in a public alert.

In Mississippi, where tornados killed 10 people nine days ago, driving rains and more tornados tore through farming regions damaging homes and destroying crops.

"We have confirmed five people dead because of the current weather condition and five people with injuries," Mississippi Emergency Management Agency spokeswoman Nikki Pressley told AFP.

She said the Red Cross and Salvation Army have evacuated dozens of people from their homes to area shelters.

In Tennessee, two days of driving rain have caused widespread flooding, killing at least 11 people in the central part of the state and forcing scores more to evacuate homes and businesses, The Nashville Tennessean reported Monday.

At least two more people were listed as missing.

The newspaper said thousands of homes were flooded and entire neighborhoods submerged across the state.

Late Sunday, Nashville announced that it was shutting down a water treatment plant and that a levee along the Cumberland River had begun to leak, the report said.

After an aerial survey, Mayor Karl Dean said the damage was worse than he thought.

"This situation is going to require a very large recovery process," the paper quotes Dean as saying. "The magnitude of the damage to our community was much more than what I expected. The safety of some of our infrastructure is questionable."

Nashville fire and rescue officials were still searching late Sunday for two young men who went missing after they tried to raft in a creek with inner tubes, the daily said.

The National Weather Service has forecast more severe weather for the next few days in the region, with several rivers at or near flood stage and nearby areas under flood warnings.