US looks for survivors amid fears Kentucky tornado death toll may top 100

US rescuers have desperately searched for survivors 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 state factory.

More than 80 people are dead in Kentucky alone, many of them workers at a candle factory in Mayfield, the state's Governor Andy Beshear said on Sunday as he raised the confirmed toll by 10 fatalities.

And the forecast was grim. "That number is going to exceed more than 100," Beshear told CNN news channel.

President Joe Biden called the wave of twisters, including one that travelled more than 320 kilometres, "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.

Brace for the worst

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 the Kentucky governor's somber remarks suggested his state's residents should brace for the worst.

Of the 110 employees working on 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," he said, but "it would be an incredible miracle" if more factory victims were found alive.

Russia sends condolences 

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.

The head of the US Federal Emergency Management Agency, Deanne Criswell, was to arrive in Kentucky Sunday.

The largest tornado among the swarm that smashed through the US South and Midwest 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.

'There is always hope'

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 on 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 leveled; historic homes and buildings beaten down to their slabs; tree trunks stripped of their branches; cars overturned in fields.