Dozens of devastating tornadoes roared through five US states overnight, leaving more than 80 people dead Saturday in what President Joe Biden said was "one of the largest" storm outbreaks in history.

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

As darkness fell Saturday scores of search and rescue officials were helping stunned citizens across the US heartland sift through the rubble of their homes and businesses searching for any more survivors. 

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.

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"This event is the worst, most devastating, most deadly tornado event in Kentucky's history," said Kentucky 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," the governor told reporters.

The western Kentucky town of Mayfield was reduced to "matchsticks," its mayor said.

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.

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

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

CNN played a heart-rending plea posted on Facebook by one of the factory's employees.

"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."

The woman, Kyanna Parsons-Perez, was rescued after being pinned under a water fountain.

- 'Like a bomb' -

"When I walked out of City Hall this morning, it -- it looked like matchsticks," Mayfield Mayor Kathy O'Nan told CNN.

"Our downtown churches have been destroyed, our courthouse... is destroyed, our water system is not functioning at this time, there is no power."

"It looks like a bomb has exploded," 31-year-old Mayfield resident Alex Goodman told AFP.

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David Norseworthy, a 69-year-old builder in Mayfield, said the storm blew off his roof and front porch while the family hid in a shelter.

"We never had anything like that here," he told AFP.

In a parking lot in downtown Mayfield, volunteers were collecting warm clothes, diapers and water for residents.

The tornado that smashed through Mayfield had rumbled along the ground for over 200 miles in Kentucky and for 227 miles overall, Beshear said. 

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

In one demonstration of the storms' awesome power on Saturday, when winds derailed a 27-car train near Earlington, Kentucky, one car was blown 75 yards up a hill and another landed on a house. No one was hurt.

- 'Pretty much destroyed' -

Reports put the total number of tornadoes across the region at around 30.

At least 13 people were killed in other storm-hit states, including at the Amazon warehouse in Illinois, bringing the total toll to 83.

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In Arkansas, at least one person died when a tornado "pretty much destroyed" a nursing home in Monette, a county official said. Another person died elsewhere in the state.

Four people died in Tennessee, while one died in Missouri.

Biden promised the full assistance of the federal government and said he planned to travel to the affected areas. 

Scientists have warned that climate change is making storms more powerful and frequent. Biden said that while the impact on these particular storms was not yet clear, "We all know everything is more intense when the climate is warming, everything."

The American Red Cross said it was working to provide relief across all five states.

Beshear declared a state of emergency in Kentucky and said scores of search and rescue officials had been deployed along with the national guard.

More than half a million homes in several states were left without power, according to PowerOutage.com.

- Amazon workers trapped -

When another tornado hit an Amazon warehouse in the southern Illinois city of Edwardsville around 100 workers were trapped inside.

Hundreds of workers scrambled to rescue the trapped employees.

"We identified 45 personnel who made it out of the building safely, one who had to be airlifted to a regional hospital for treatment, and six fatalities," Edwardsville, Illinois fire chief James Whiteford told a press conference.

Amazon spokesperson Richard Rocha said its workers' safety was the company's "top priority."