Amazon chief Jeff Bezos said he was "heartbroken" Saturday after at least six people were killed at an Amazon warehouse in the US state of Illinois when it was struck by a tornado.

"The news from Edwardsville is tragic," Bezos tweeted about the town where the facility was located.

"We're heartbroken over the loss of our teammates there, and our thoughts and prayers are with their families and loved ones."

The warehouse was struck by a series of tornadoes that ripped across six US states, killing more than 80 people in total in what President Joe Biden said was likely to be "one of the largest" storm outbreaks in history.

Up to 100 Amazon employees working a night shift ahead of Christmas are believed to have been trapped when the warehouse collapsed, and it is not clear how many may still be trapped.

Earlier Saturday, Edwardsville fire chief James Whiteford told reporters that 45 people made it out of the building safely, one had to be airlifted to hospital for treatment, and at least six had died.

But he said the operation had turned from rescue to focus "only on recovery," fueling fears the toll could yet rise.

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

"We extend our fullest gratitude to all the incredible first responders who have worked so tirelessly at the site."