NASHVILLE, Tennessee (Reuters) - Powerful storms that spawned tornadoes ripped through the U.S. Midwest on Wednesday, killing at least 12 people, including six in Illinois who were crushed when a house was lifted up and fell on them, authorities said.

The violent weather that hit six Midwest states starting in Kansas and Missouri overnight swept into middle Tennessee and slammed the Cumberland Plateau region, about an hour east of Nashville, killing two women in Cumberland County and one person in DeKalb County, according to emergency agency officials.

Three people in Missouri were also killed by a storm that struck during the night. A tornado temporarily closed the famous entertainment strip in Branson, Missouri, where country music shows and other performers draw thousands of people a day.

Two men and four women died when a pre-dawn tornado struck Harrisburg, Illinois, a town of nearly 10,000 people, Mayor Eric Gregg said, describing the storm damage as "horrific."

In Harrisburg, the six people were killed when powerful winds lifted a house up and dropped it on top of other homes in a housing subdivision adjacent to a wrecked shopping strip.

"There are hundreds of homes damaged, millions of dollars in damage. The hospital is severely damaged. There's a mall with 10 stores that was destroyed," Gregg said.

Mike Hancock, an employee of the U.S. Forest Service, and several others armed with tools attempted a rescue where the six people died.

"We crawled in there as much as we could. Then there wasn't enough stability, the whole foundation was shaking. We had to get out of there," he said.

Illinois Governor Pat Quinn issued a disaster declaration for the southern third of the state, Missouri Governor Jay Nixon declared a state of emergency for the state, while Kansas Governor Sam Brownback  declared a state of disaster emergency for Wabaunsee County.

The National Weather Service's Storm Prediction Center rated the Harrisburg tornado an EF-4, or one notch below the strongest tornadoes, meaning it packed winds of up to 200 miles per hour. The EF-4 rating put it on par with the devastating tornado that killed 64 people in Tuscaloosa, Alabama, last April, and one notch below the massive EF-5 Joplin storm that flattened whole sections of the Missouri town.

The violent weather prompted reports of 18 tornadoes across six states, including Nebraska, Kansas, Missouri, Illinois, Indiana and Kentucky, according to the National Weather Service.

Forecasters said there was no relief in sight as the stormy weather headed east to the Mid-Atlantic states and parts of the Southeast.

"We have a number of strong, severe storms ongoing from the Appalachian Mountains through the Southeast, said meteorologist Jared Guyer of the National Weather Service Storm Prediction Center.

There were tornado watches issued for parts of Mississippi, Alabama, Tennessee, Kentucky, Virginia, West Virginia and Ohio.