US emergency workers on Sunday desperately searched for survivors of ferocious late-season tornadoes that killed at least 94 across several states and left towns in ruins, including in the debris of a Kentucky candle factory, a symbol of the widespread devastation.

But the state's governor Andy Beshear warned that the search in that facility in Mayfield -- a town almost completely wiped out by the twisters -- may be in vain, admitting "another rescue" may not happen there.

President Joe Biden has called the wave of tornadoes "one of the largest" storm outbreaks in American history, adding, "We still don't know how many lives are lost and the full extent of the damage."

He sent the heads of the Homeland Security Department and the Federal Emergency Management Agency (FEMA) to Kentucky to assess the situation, and promised the full gamut of federal aid.

With the death count 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.

More than 80 people are dead in Kentucky alone, many of them workers at the Mayfield factory, the Beshear said Sunday as he raised the confirmed toll by 10 fatalities.

"That number is going to exceed more than 100," Beshear told CNN.

Of the 110 employees working Friday night in the candle factory, "about 40 of them have been rescued and I'm not sure we're going to see another rescue," Beshear said.

"I pray for it. It would be an incredible miracle."

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

Emergency crews worked through the night into Sunday at both locations, and FEMA agents and Red Cross volunteers were on the scene in Kentucky.

But Edwardsville fire chief James Whiteford -- giving the toll at six dead and 45 safely out of the building -- told reporters the operation had turned from rescue to focus "only on recovery," fueling fears the toll will rise.

'Unlike anything I have seen'

The storm system's power placed it in historic company.

Storm trackers said it had lofted debris 30,000 feet (9,100 meters) in the air, and the deadly Mayfield twister appeared to have broken an almost century-old record, tracking on the ground well more than 200 miles (320 kilometers).

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.

The catastrophe has shaken many Americans, including officials who have worked through other big storms.

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

"The devastation is unlike anything I have seen in my life."

'Hope for a miracle'

Mayfield, a town of about 10,000 near the westernmost tip of Kentucky, was reduced to "matchsticks," its mayor Kathy O'Nan said.

Still, she told NBC on Sunday, "there's always hope" of finding survivors among the missing.

"We hope for a miracle."

But as a nondenominational church in Mayfield was handing out food and clothing to storm survivors, it was also providing space for the county coroner to do his work, pastor Stephen Boyken of His House Ministries told AFP.

People "come with pictures, birthmarks -- they talk now about using DNA samples to identify those who have been lost," he said.

Troy Propes, CEO of the company that owned the candle factory, defended his decision not to close it as the storm neared.

"We did everything that was supposed to happen," he told CNN on Sunday. "My heart bleeds for absolutely everyone."

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

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

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.

Deaths in multiple states

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

At least 14 people were killed in other storm-hit states, including six at the Amazon facility in Illinois.

Four were killed in Tennessee, two died in Arkansas, while Missouri recorded two fatalities. Tornadoes also touched down in Mississippi.

Biden said he planned to travel to the affected areas.

"This is going to be our new normal. And the effects that we're seeing from climate change are the crisis of our generation," Deanne Criswell, the administrator of the Federal Emergency Management Agency, told CNN on Sunday.