A massive spring storm that has swamped the central United States with days of heavy rain and deadly twisters threatened more destruction Wednesday as the death toll rose to 24, officials said.

Dramatic rescues were caught on camera as crews braved rushing waters to pluck people stranded by flash flooding.

Roads were washed out or rendered impassable by fallen trees and power lines. Houses, schools and businesses were flattened and flooded. Debris tossed into the air by the high winds rained down on soaking streets.

Governors in several states called out the National Guard to help with rescue and cleanup operations and states of emergency were declared in Missouri, Alabama, Arkansas and Kentucky.

"Our citizens have endured days of consecutive severe storms and flooding," said Kentucky Governor Steve Beshear.

"We stand ready to assist any community in need, and we urge Kentuckians to remain on alert until this storm system finally passes."

Hundreds of people were evacuated from their homes in Missouri after levees failed to hold back swollen rivers.

"I'm just glad my family is safe," said Chris Pigg, who spent the night at a shelter with his wife and daughter and wasn't sure if he'd have a home to return to after the Black River breached the levee in Poplar Bluff, Missouri.

The National Weather Service issued a rare "high-risk" warning of tornadoes, hail, flash flooding and dangerous lightning for parts of Alabama, Georgia, Mississippi and warned that severe weather could also strike 21 states from the Great Lakes down to the Gulf Coast and across to the Atlantic.

More than 50 tornadoes were reported on Tuesday alone and there have been hundreds of reports of wind damage.

The skies are not expected to clear until late Thursday or Friday.

There will be little time to mop up, as another major storm system is forecast to bring heavy rain and high winds on Saturday.

The storm comes after a wet spring and a winter of heavy snowfall, which means the ground is saturated and rivers were already quite high.

Officials were considering deliberately destroying levees in some areas to ease pressure on swollen rivers, some of which are so high that barges have become trapped under bridges.

As much as 18 inches (45 centimeters) of rain had fallen from Saturday through Tuesday night in some areas.

"It's producing major to record flooding in a lot of those river basins," said Jim Keeney, deputy chief of the weather service's central region.

Arkansas has been among the hardest hit as it deals with flash flooding and a series of tornado strikes that have killed 11 people.

"It looks like a tornado came through the area because that's what happened," Renee Preslar, a spokeswoman for the Arkansas department of emergency management, told AFP.

"There's trees down, the power's down, you've got homes that are damaged... as well as a lot of flooding going on."

Six deaths were reported in Alabama, five people were killed and 30 were hurt by the storm in Mississippi, and two more were killed by floodwaters in Missouri, officials said.

Officials across the country begged people to use common sense and not to try to drive or walk through the floodwaters.

"Some people are still trying to drive around barricades with their cars. Please don't do that," Louisville, Kentucky Mayor Greg Fisher said at a press conference.

"If it's moving fast you can go with it."