Tornadoes ripped across a wide swath of central and southern United States on Wednesday, destroying homes and businesses and bringing down power lines and trees.

The National Weather Service (NWS) said there had been at least 15 reports of tornadoes in at least four states by late Wednesday.

Eight people have been injured across Kentucky and Arkansas, including one critically injured in Kentucky's Ballard County, local officials said.

Late Wednesday, Arkansas Governor Sarah Huckabee Sanders declared a state of emergency across the state due to the storms, which also brought hail and torrential rain.

The NWS said millions of people were under alerts for tornadoes and flash floods and that dangers would continue into early Thursday.

Violent storms are forecast to ravage the country for several days, the NWS said, with Wednesday just "the beginning of a multi-day catastrophic and potentially historic heavy rainfall event."

The NWS issued tornado and flash flood warnings for parts of Missouri, Arkansas, Tennessee, Mississippi, Indiana, Illinois, Kentucky and Oklahoma.