US President Joe Biden has approved an emergency declaration for Kentucky after the state was battered by a swarm of tornadoes that have killed at least 100 people.

The devastating twisters ripped through six US states, leaving a trail of destroyed homes and businesses along a path that stretched more than 320km, officials said on Saturday.

Biden has ordered federal aid to supplement the response from state and local authorities, the White House said.

“It’s a tragedy. And we still don’t know how many lives were lost and the full extent of the damage,” Biden told reporters.

Kentucky Governor Andy Beshear said the collection of tornadoes was the most destructive in the state’s history.

The loss of life at one candle factory in the town of Mayfield had the highest fatality count at one location of any tornado in Kentucky’s history, he said.

“There were about 110 people in it (the candle factory) at the time that the tornado hit it,” Mr Beshear said.

“We believe we’ll lose at least dozens of those individuals. It’s very hard, really tough, and we’re praying for each and every one of those families.”

The most intense damage I have found in #Mayfield. The entire path in the town is less than 2% of the total path length. Homes with only slabs remaining. #Tornado #wxtwitter pic.twitter.com/nXxoxNBVhY

— WxChasing- Brandon Clement (@bclemms) December 11, 2021

Mr Beshear said the destruction in Kentucky was “indescribable, unlike anything I’ve ever seen”.

“You see parts of industrial buildings, roofs, or sightings in trees, if trees are lucky enough to stand,” he said.

“Huge metal poles bent in half if not broken, buildings that are no longer there, huge trucks that have been picked up and thrown. And sadly far too many homes that people were likely in, entirely devastated.

“We have half of some of our towns here in western Kentucky that are just gone.”

Emergency workers search what is left of the Mayfield Consumer Products Candle Factory. Photo: Getty

Mr Beshear said 189 National Guard personnel have been deployed to assist with the recovery, with a focus on Mayfield, a small city of about 10,000 people.

Fire and police stations in Mayfield were destroyed, hindering the emergency response.

Kentucky Gov. Andy Beshear says the death toll in Kentucky may reach 100 after a strong tornado blew through at least six states last night: 'We are hurting this morning' pic.twitter.com/zE2ibJFl8a

— Markets Today (@marketsday) December 11, 2021

Bunkered down in the candle factory which later collapsed, Kyanna Parsons-Perez told CNN staff had been moved to a shelter area when the tornado struck the building.

“We felt the wind. My ears started popping, and then it was like the building, like we all just rocked back and forth, and then boom, everything fell on us,” she told CNN.

“It was the most terrifying thing that I have ever experienced in my life.”

She added: “At first, I was really calm, and I was trying to keep my co-workers calm … but then, after being pinned down for so long, and my legs were hurting and I couldn’t move them and I couldn’t feel them and stuff like that, I started to panic myself.”

This pretty much sums it up in :19. #Mayfield #MayfieldTornado #KyWx #WX pic.twitter.com/IcPL4XGtPS

— WxChasing- Brandon Clement (@bclemms) December 11, 2021

Around five deaths have so far been reported in other states.

Police in Edwardsville, Illinois, confirmed fatalities at an Amazon facility where the roof was ripped off the building and a wall about the length of a football field collapsed.

Three storm-related deaths were confirmed in Tennessee, according to the Tennessee Emergency Management Agency.

And a tornado struck the Monette Manor nursing home in Arkansas on killing one person and trapping 20 people inside as the building collapsed, local county judge Marvin Day said.

One person died and two others were injured in building collapses near the towns of Defiance and New Melle.

Cars flipped over and buildings turned to splinters in Mayfield, Kentucky. Photo: Getty

US president Joe Biden tweeted that he had been briefed on the tornadoes.

“To lose a loved one in a storm like this is an unimaginable tragedy,” he wrote.

“We’re working with Governors to ensure they have what they need as the search for survivors and damage assessments continue.”

The National Weather Service tweeted that the storm from this weather event tracked more than 250 miles through several states.

“One or more deadly tornadoes were spawned, with major structural damage noted,” said the tweet.

-with AAP