Illinois Gov. Pat Quinn declared seven counties as disaster areas.

Dozens of tornadoes and intense thunderstorms swept across the U.S. Midwest on Sunday, leaving at least six people dead and unleashing powerful winds that flattened entire neighborhoods, flipped over cars and uprooted trees.

An elderly man and his sister were killed when a tornado hit their home in rural southern Illinois. Four other people were killed in the state, the hardest hit by the tornados, said Patti Thompson of the Illinois Emergency Management Agency. She did not provide details.

Illinois Gov. Pat Quinn declared seven counties as disaster areas.

With communications difficult and many roads impassable, it remained unclear how many people were killed or hurt by the unusually strong late-season tornadoes.

Between 250 and 500 homes were either damaged or destroyed in the town of Washington, Mayor Gary Manier said on Monday. He said it wasn’t clear when residents would be allowed to return.

“Everybody’s without power, but some people are without everything,” Mr. Manier told reporters in the parking lot of a destroyed auto parts store and near a row of flattened homes.

“How people survived is beyond me,” he said.

The tornado cut a path from one side of a town of 16,000 people to the other, knocking down power lines, uprooting trees and rupturing gas lines, State Trooper Dustin Pierce said.

Local official Tyler Gee told WLS—TV that as he walked through neighborhoods immediately after the tornado struck, he “couldn’t even tell what street I was on.”

“Just completely flattened some of the neighborhoods here in town, hundreds of homes.”

At OSF Saint Francis Medical Center in Peoria, spokeswoman Amy Paul said 37 patients had been treated, eight with injuries ranging from broken bones to head injuries that were serious enough to be admitted. Another hospital, Methodist Medical Center in Peoria, treated more than a dozen people, but officials there said none were seriously injured.

As the rain and high winds slammed into the Chicago area, officials cleared a professional sports stadium and cleared teams off the field for a couple of hours.

Just how many tornadoes hit was unclear. According to the National Weather Service’s website, a total of 65 tornadoes struck, most of them in Illinois. But meteorologist Matt Friedlein said the total might fall because emergency workers, tornado spotters and others often report the same tornado.

The storm system was pushing into the mid,Atlantic and Northeastern states on Sunday evening.

More In: World | International | News