Night of devastating tornadoes kills more than 100 in Kentucky

At least 100 people were feared dead in Kentucky after a swarm of tornadoes tore through the US Midwest and South, demolishing homes, levelling businesses and setting off a scramble to find survivors beneath the rubble.

The powerful twisters, which weather forecasters say are unusual in cooler months, destroyed a candle factory and the fire and police stations in a small town in Kentucky, ripped through a nursing home in neighbouring Missouri, and killed at least six workers at an Amazon warehouse in Illinois.

Kentucky Governor Andy Beshear said the collection of tornadoes was the most destructive in the state's history. He said about 40 workers had been rescued at the candle factory in the city of Mayfield, which had about 110 people inside when it was reduced to a pile of rubble. It would be a "miracle" to find anyone else alive under the debris, Beshear said.

"The devastation is unlike anything I have seen in my life and I have trouble putting it into words," Beshear said at a press conference. "It's very likely going to be over 100 people lost here in Kentucky."

Beshear said 189 National Guard personnel have been deployed to assist with the recovery. The rescue efforts will focus in large part on Mayfield, home to some 10,000 people in the southwestern corner of the state where it converges with Illinois, Missouri and Arkansas.

Video and photos posted on social media showed brick buildings in downtown Mayfield flattened, with parked cars nearly buried under debris. The steeple on the historic Graves County courthouse was toppled and the nearby First United Methodist Church partially collapsed.

Mayfield Fire Chief Jeremy Creason, whose own station was destroyed, said the candle factory was diminished to a "pile of bent metal and steel and machinery" and that responders had to at times "crawl over casualties to get to live victims".

The genesis of the tornado outbreak was a series of overnight thunderstorms, including a super cell storm that formed in northeast Arkansas. That storm moved from Arkansas and Missouri and into Tennessee and Kentucky.

Unusually high temperatures and humidity created the environment for such an extreme weather event at this time of year, said Victor Gensini, a professor in geographic and atmospheric sciences at Northern Illinois University.

Saying the disaster was likely one of the largest tornado outbreaks in U.S. history, President Joe Biden on Saturday approved an emergency declaration for Kentucky.

He told reporters he would be asking the Environmental Protection Agency to examine what role climate change may have played in fuelling the storms, and he raised questions about the tornado warning systems.

"What warning was there? And was it strong enough and was it heeded?" Biden said.