President Joe Biden called the wave of tornadoes, including one that travelled more than 200 miles, "one of the largest" storm outbreaks in American history.

"It's a tragedy," a shaken Biden, who pledged support for the affected states, said in televised comments. "And we still don't know how many lives are lost and the full extent of the damage."

Scores of search and rescue officials were helping stunned citizens across the US heartland sift through the rubble of their homes and businesses overnight. 

More than 70 people are believed to have been killed in Kentucky alone, many of them workers at a candle factory, while at least six died in an Amazon warehouse in Illinois where they were on the night shift processing orders ahead of Christmas.

"This event is the worst, most devastating, most deadly tornado event in Kentucky's history," said state governor Andy Beshear, adding he fears "we will have lost more than 100 people."

"The devastation is unlike anything I have seen in my life, and I have trouble putting it into words," he told reporters.

Beshear has declared a state of emergency.

The tornado that smashed through Kentucky had rumbled along the ground for over 200 miles (320 kilometers), Beshear said, one of the longest on record.

The longest a US tornado has ever tracked along the ground was a 219-mile storm in Missouri in 1925. It claimed 695 lives.

'Like a bomb'

The western Kentucky town of Mayfield was reduced to "matchsticks," its mayor Kathy O'Nan told CNN.

The small town of 10,000 people was described as "ground zero" by officials, and appeared post-apocalyptic: city blocks leveled; historic homes and buildings beaten down to their slabs; tree trunks stripped of their branches; cars overturned in fields.

Some Christmas decorations could still be seen by the side of the road.

Beshear said there were 110 people working at the candle factory when the storm hit, causing the roof to collapse. 

Forty people have been rescued, but it would be "a miracle if anybody else is found alive," he said.

CNN played a heart-rendering plea posted on Facebook by a factory employee.

"We are trapped, please, y'all, get us some help," a woman says, her voice quavering as a co-worker can be heard moaning in the background.

"We are at the candle factory in Mayfield... Please, y'all. Pray for us."

AFP/eNCA