US President Joe Biden on Saturday authorized federal aid for Kentucky after at least 70 people died when devastating tornadoes tore through six US states late Friday night.

The twisters, which could be the worst on record, reduced buildings to rubble and could have killed "over 100 people" just in Kentucky, said the state's governor Andy Beshear. A total of 36 deaths from 5 US states have so far been officially confirmed.

Biden laments tornado 'tragedy'

Created out of powerful 'supercell' thunderstorms that formed in the US state of Arkansas the tornadoes moved to Tennessee and Kentucky wreaking devastating damage on everything in their path.

US President Joe Biden said it was "one of the largest tornado outbreaks in our history."

He approved an emergency declaration for Kentucky, the White House said.

"It's a tragedy. And we still don't know how many lives are lost and the full extent of the damage," Biden said on television. He promised assistance to all six states impacted by the natural disaster.

Biden said the affected states would "have what they need as the search for survivors and damage assessments continue."

He said he would be asking the Environmental Protection Agency to investigate whether climate change contributed to the extreme weather.

Biden also questioned tornado warning systems after the US National Weather Service issued its highest level of alert on Friday night for the states of Kentucky, Arkansas, Tennessee, Missouri and Illinois.

"What warning was there? And was it strong enough and was it heeded?" he asked.

'Unlike anything I have ever seen'

Officials declared the western Kentucky town of Mayfield "ground zero" after tornadoes leveled city blocks, destroyed historic homes, toppled trees and overturned cars.

Mayfield mayor Kathy O'Nan said "it looked like matchsticks" on Saturday.

The governor of Kentucky said Saturday that the collection of tornadoes was the most destructive in the state's history. 

"The level of devastation is unlike anything that I have ever seen... We were pretty sure that we would lose over 50 Kentuckians, I'm now certain that that number is north of 70. It may in fact end up exceeding 100 before the day is done," Beshear told reporters.

According to Beshear, 189 National Guard personnel have been deployed to assist with the emergency response.

The tornado also derailed a 27-car train, even throwing one of the cars up a hill and onto a house. No-one was hurt.

"This has been one of the toughest nights in Kentucky history and some areas have been hit in ways that is hard to put into words," he said. 

The governor said he had declared a state of emergency.

Dozens of factory workers missing

In Mayfield, some 110 people were inside a candle factory when the tornado struck, taking down the roof, Beshear said. At least 40 of them have been rescued, he added.

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."

Emergency teams were using heavy machinery to remove rubble at the factory, said Kentucky State Trooper Sarah Burgess, adding it could take a day or more.

Burgess said coroners were on the scene to recover bodies, but she did not know how many they had found.

Amazon warehouse partially collapses

In Edwardsville, Illinois, the tornado killed at least six workers at a warehouse of online retail giant Amazon, reducing a third of the structure to rubble. The roof of the structure was ripped off and one of its walls collapsed. 

Rescue workers were rushing through the early hours of Saturday to rescue employees who were working the night shift when the tornado hit. 

Local authorities described the warehouse collapse as a "mass casualty incident" with "multiple subjects trapped."

Although 45 workers were saved, hope was dwindling for the rest of the workers in the 500,000 square foot (46,451 square meter) building.

Amazon founder Jeff Bezos tweeted how he was "heartbroken" at the "tragic" loss of life at the company's facility.

"All of Edwardsville should know that the Amazon team is committed to supporting them and will be by their side through this crisis," Bezos added.

The Retail, Wholesale and Department Store Union criticized Amazon for keeping the facility open during a weather emergency.

In Arkansas, a twister struck a nursing home in Monette, killing one and seriously injuring five others, while in Missouri two people died and hundreds of homes were flattened.

Tornado could be 'longest' on record, says expert

A twister touched down for more than 320 kilometers (200 miles) in Kentucky, "making it one of the longest track violent tornadoes in United States history,'' said Victor Gensini, a researcher on extreme weather at Northern Illinois University.

The longest tornado on record tore through Missouri, Illinois for about 355 kilometers (220 miles).

Tornadoes are created when denser, drier cold air is pushed over warmer, humid air. An updraft is created as the warm air rises, and the spinning starts when wind varies at speed at different altitudes.

Normally tonadoes last for a few minutes but this one went on for hours.

Gensini said this twister, which is rare in December, could have combed the ground for as much as 400 kilometers (250 miles). "This is an historic, if not generational event," Gensini added.

Scientists are unsure if there was just one tornado or several that caused the destruction.

Meanwhile in Tennessee, the severe weather killed at least three people, the Washington Post quoted a spokesman for the state's Emergency Management Agency as saying. 

mvb,jcg,jc/fb,wd (AP, AFP, Reuters)