US rescuers desperately searched for survivors Sunday after tornadoes killed at least 94 and left towns in ruin, with emergency crews racing against time to find dozens still missing from a collapsed Kentucky factory.

President Joe Biden called the wave of twisters, 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."

With the death toll all but certain to rise, scores of search and rescue officers 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.

Meanwhile, at least six died in an Amazon warehouse in Illinois where they were on the night shift processing orders ahead of Christmas.

Emergency crews worked through the night into Sunday at both locations, but a Kentucky official's sombre remarks suggested his state's residents should brace for the worst.

"There's been no new news," Kentucky emergency management director Michael Dossett told CNN. "It is a very sad and solemn operation at this point."

As Americans grappled with the immensity of the disaster, condolences poured in, with Pope Francis saying he is praying "for the victims of the tornado that hit Kentucky."

Biden's Russian counterpart Vladimir Putin, in a break from tense bilateral relations, said his country "shares in the grief" of those who lost loved ones and expressed hope that victims quickly overcome the tornadoes' consequences.

The catastrophe has shaken many Americans, including officials who have worked in the aftermath of tornadoes and other big storms.

"This event is the worst, most devastating, most deadly tornado event in Kentucky's history," Kentucky Governor Andy Beshear said Saturday, 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."

Beshear has declared a state of emergency, and he said the head of the US Federal Emergency Management Agency arrives in Kentucky Sunday.

The tornado that smashed through Kentucky had rumbled along the ground for over 320 kilometres, 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.

'Hope for a miracle'

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

"There's always hope" for survivors among the missing, O'Nan told NBC Sunday. "We hope for a miracle in the days to come."

The town of 10,000 was described as "ground zero" by officials, and appeared post-apocalyptic: city blocks levelled; 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.

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 one demonstration of the storms' power 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 an Amazon warehouse in Illinois, bringing the total toll to 83.

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. Tornadoes also touched down in Mississippi.

Biden said he planned to travel to the affected areas. 

Amazon workers trapped

One of the tornadoes hit the Amazon warehouse in the southern Illinois city of Edwardsville, with around 100 workers believed to have been trapped inside.

"We identified 45 personnel who made it out of the building safely... and six fatalities," Edwardsville fire chief James Whiteford told reporters.

But he said the operation had turned from rescue to focus "only on recovery," fuelling fears the toll will rise.

Amazon chief Jeff Bezos said he was "heartbroken" at the deaths.