Dozens of devastating tornadoes have ripped through five US states overnight, leaving more than 80 people dead on 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 levelled, 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."
Mr Beshear said it was clear that the death toll in his state 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 mid afternoon that there had been no successful rescues from the factory since 3am local time, 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 320km in Kentucky and for 365km overall, Mr Beshear said.
Swaths of deadly tornadoes have torn through the US Midwest. Source: Getty Images North America
Previously, the longest a US tornado has ever tracked along the ground was a 350km 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 almost 70 metres up a hill, and another landed on a house, though no one was hurt.
Reports put the total number of tornadoes across the region at around 30, and 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.
'Unimaginable,' says Joe Biden
In televised comments, President Joe Biden said the powerful storm system is likely to be "one of the largest tornado outbreaks in our history".
"It's a tragedy. And we still don't know how many lives are lost and the full extent of the damage," he said.
The American Red Cross said it was working to provide relief across all five states and noted a continued threat of more severe weather.
Images of the latest tornadoes from US news channels showed dark black cylinders sweeping across the ground, illuminated by intermittent flashes of lightning.
Multiple tornadoes tore through parts of the lower Midwest late on Friday night leaving a large path of destruction. Source: Getty Images North America
At least four Kentucky counties were left devastated, with officials describing Mayfield as "ground zero."
"Mayfield in Graves County will be ground zero," Kentucky emergency management director Michael Dossett told CNN. "There is massive devastation".
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, said police quoted by local media.
Hundreds of workers scrambled to rescue the trapped or injured employees, who were on the night shift processing orders ahead of the Christmas holidays.
Amazon chief Jeff Bezos said he was "heartbroken" after at least six people were killed at the warehouse.
"The news from Edwardsville is tragic," Mr Bezos tweeted about the town where the facility was located.
"We're heartbroken over the loss of our teammates there, and our thoughts and prayers are with their families and loved ones."
Scientists have warned that climate change is making storms more powerful and increasing their frequency, posing a growing threat to areas where extreme weather events are already common.
Tornado outbreaks in this region are rare in December, however, according to Sam Shamburger, the lead NWS forecaster in Nashville, Tennessee. The La Nina weather phenomenon may be aggravating normal weather patterns, he said.
"We could see even more of these severe weather-type systems even during the winter months, December, January, February," Tennessee news channel WKRN quoted him as saying.
"Stay alert over the next few months."
