Dozens of devastating tornadoes ripped through five US states overnight, leaving more than 70 people dead Saturday in Kentucky -- many of them workers at a candle factory -- and inflicting deadly damage at a sprawling Amazon warehouse in Illinois.

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

The small town of 10,000 people appeared post-apocalyptic in drone footage posted by storm-chaser Brandon Clement: city blocks leveled, with almost nothing salvageable; historic homes and buildings beaten down to their slabs; tree trunks stripped of their branches; cars overturned in fields.

"It is indescribable -- the level of devastation is unlike anything I have ever seen," Kentucky governor Andy Beshear said after rushing to Mayfield. "This will be, I believe, the deadliest tornado system to ever run through Kentucky."

Beshear said it was clear that the death toll in his state was already "north of 70" and could end up "exceeding 100 before the day is done."

Referring to the candle factory, where a roof collapsed, he said: "We're going to lose a lot of lives in that facility. It's a very dire situation."

CNN played a heart-rending tape of an urgent 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.

But Mayfield Mayor Kathy O'Nan told CNN in midafternoon that there had been no successful rescues from the factory since 3:00 am, adding to fears the death toll will rise.

"When I walked out of City Hall this morning, it -- it looked like matchsticks," she said of Mayfield.

"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 in our community," 31-year-old Mayfield resident Alex Goodman told AFP. "The sheer force of the wind and the rain was incredible."

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. Powerful and devastating -- as such long-track storms tend to be -- it claimed 695 lives.

In one measure of this storm's awesome power, 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, though no one was hurt.

'Unimaginable,' says Biden

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

At least eight people were killed in other storm-hit states, including two at the Amazon facility in Illinois.

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.

President Joe Biden spoke to the governors of all five states and said the massive storm system had inflicted an "unimaginable tragedy". He vowed to provide all needed federal aid.

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

At least four Kentucky counties were left devastated, with officials describing Mayfield as "ground zero."

The governor declared a state of emergency 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, reducing much of it to rubble, around 100 workers were trapped inside.

Hundreds of workers scrambled to rescue the trapped or injured employees, who were on the night shift processing orders ahead of Christmas. Two died, local media reported.

Amazon spokesperson Richard Rocha said its workers' safety was the company's "top priority," adding, "We're assessing the situation and will share additional information when it's available."

Scientists have warned that climate change is making storms more powerful and increasing their frequency.

Tornado outbreaks in this region are rare in December, however.