"It's a tragedy. And we still don't know how many lives are lost and the full extent of the damage," he said in televised comments, quoted by AFP.

At least 78 people were killed, with more than 70 of them in Kentucky alone, after the series of tornadoes roared across five states, leaving post-apocalyptic scenes of devastation.

There are fears the toll will rise.

"Whatever is needed, I'm going to ask for," Biden vowed at his press conference.

When asked whether climate change had impacted the storms, he said he "can't say" and that he would ask the government's Environmental Protection Agency to look in to the question.

"But the fact is that we all know everything is more intense when the climate is warming, everything," Biden continued. "And obviously it has some impact here but I can't give you a quantitative read on that." 

He promised to visit the damaged region, but said he wanted to be sure he did not get "in the way of rescue and recovery."

"But I ... will plan on going," he said.

Reuters reported at least 100 people were feared dead in Kentucky after a swarm of tornadoes tore a 200-mile path through the US Midwest and South, demolishing homes, levelling businesses and setting off a scramble to find survivors beneath the rubble, officials said Saturday.

The powerful twisters, which weather forecasters say are unusual in cooler months, destroyed a candle factory and the fire and police stations in a small town in Kentucky, ripped through a nursing home in neighboring Missouri, and killed at least six workers at an Amazon warehouse in Illinois.

Kentucky Governor Andy Beshear said the collection of tornadoes was the most destructive in the state's history. He said about 40 workers had been rescued at the candle factory in the city of Mayfield, which had about 110 people inside when it was reduced to a pile of rubble. It would be a "miracle" to find anyone else alive under the debris, Beshear said.

"The devastation is unlike anything I have seen in my life and I have trouble putting it into words," Beshear said at a press conference. "It's very likely going to be over 100 people lost here in Kentucky."

Beshear said 189 National Guard personnel have been deployed to assist with the recovery. The rescue efforts will focus in large part on Mayfield, home to some 10,000 people in the southwestern corner of the state where it converges with Illinois, Missouri and Arkansas.

Video and photos posted on social media showed brick buildings in downtown Mayfield flattened, with parked cars nearly buried under debris. The steeple on the historic Graves County courthouse was toppled and the nearby First United Methodist Church partially collapsed.

Mayfield Fire Chief Jeremy Creason, whose own station was destroyed, said the candle factory was diminished to a "pile of bent metal and steel and machinery" and that responders had to at times "crawl over casualties to get to live victims."

Paige Tingle said she drove four hours to the site in the hope of finding her 52-year-old mother, Jill Monroe, who was working at the factory and was last heard from at 9:30 p.m.

"We don't know how to feel, we are just trying to find her," she said. "It's a disaster here."

The genesis of the tornado outbreak was a series of overnight thunderstorms, including a super cell storm that formed in northeast Arkansas. That storm moved from Arkansas and Missouri and into Tennessee and Kentucky.

Unusually high temperatures and humidity created the environment for such an extreme weather event at this time of year, said Victor Gensini, a professor in geographic and atmospheric sciences at Northern Illinois University.

"This is an historic, if not generational event," Gensini said.

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