Tornado outbreak likely 'one of the largest' in US history

President Joe Biden has said the powerful storm system that devastated parts of the United States overnight 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 in televised comments.

At least 80 people have been 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 (EPA) to look into 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 is promising to visit the damaged region, but says he wants to be sure he does not get "in the way of rescue and recovery."

READ MORE: Dozens dead as powerful tornadoes hit US state of Kentucky

Unknown number of missing workers

At least six people were killed when a tornado hit an Amazon warehouse in Illinois, firefighters said Saturday, bringing the death toll from storms that devastated parts of the United States overnight to more than 80.

"We identified 45 personnel who made it out of the building safely, one who had to be airlifted to a regional hospital for treatment, and six fatalities," Edwardsville, Illinois fire chief James Whiteford told a press conference.

The fire chief says the number of missing warehouse workers is unknown and they don't expect survivors.

On the other side, a FedEx spokesperson said on Saturday as part of the company's contingency plans to lessen the impact on service, co diverted several flights to other cities during the storms.

Severe weather across the central part of the US last night affected FedEx express operations at the company's Memphis hub in Tennessee.

Transport company's crew are safe, the spokesperson added.