At least six Amazon workers have been killed at a warehouse in Edwardsville, Illinois, when a series of tornadoes roared through the area, ripping off the roof and reducing a wall longer than a football field to rubble, Reuters reports.

Several workers remained unaccounted for as rescuers searched for anyone who might still be trapped at the facility, authorities said.

Local fire chief James Whiteford said 45 people had made it out safely as of Saturday afternoon. He said six people were confirmed killed so far, and he expected recovery efforts to last for at least three more days.

Fillback said earlier that about 50 workers were believed to be at the warehouse when it was hit by an extreme "weather-related event" about 8:30 p.m. Friday.

At least 30 workers were transported by bus from the scene and others may have left on their own, Fillback added. He said it had been difficult to get an exact number of workers who might be unaccounted for because the warehouse did not have "a set staff."

The tornadoes that hit the warehouse were part of a swarm of tornadoes which ripped through six US states.

Kentucky Governor Andy Beshear said on Saturday that 70 people are likely to have died in the state from the tornadoes, but that the death toll may have already reached 100.

Later on Saturday, US President Joe Biden spoke to reporters about the disaster relief efforts following the tornadoes in Kentucky.

"I have been in touch with the governors, the mayors, and FEMA, and have approved the declaration of a state of emergency requested in Kentucky," he commented. "In this respect, we are not Democrats or Republicans, but simply the United States of America. I have instructed FEMA to ensure that local and state governments know what assistance is available for them, and to make sure that they get it."

Biden clarified that calling in the National Guard is a possibility and that he will ask Congress to approve additional relief efforts. "We will do whatever is needed, provided the states conclude they need it. The National Guard has been called out in one state so far. I'll ask Congress to approve whatever is needed."