One car ended up on top of a destroyed house

Some 15 houses were levelled in the town of Stoughton, which appeared to be one of the hardest-hit areas.

The US National Weather Service said it was investigating reports of 18 possible tornadoes across the state.

"The sky just exploded. It was debris everywhere," witness David Murray told the Associated Press news agency.

"When it went across the road and it hit all the houses over there... it was something you can't explain," he said.

Some 80 houses were damaged across the state, and witnesses spoke of cars being tossed about like toys.

Governor Jim Doyle declared a state of emergency in Dane and Richland counties in southern Wisconsin.

A 54-year-old man apparently died of injuries when the storms destroyed his house near Stoughton on Thursday evening, officials said.

Several witnesses captured the tornadoes on their camera phones and camcorders.