“Multiple explosions occurred today at US Steel Clairton Coke Works,” the state’s governor Josh Shapiro said in a post on X, naming a plant some 25km outside the city of Pittsburgh.

“Injured employees have now been transported to local hospitals to receive care, and search-and-rescue efforts remain active at the plant,” he added.

At least two people were believed to be missing, said Allegheny County Police, which confirmed on Facebook there had been one fatality.

It added that “multiple individuals have been treated for additional injuries. The status of those individuals is not known at this time.”

Senator John Fetterman of Pennsylvania posted on X that “dozens” had been wounded.

US Steel said the incident happened at around 11am (1500 GMT) on Monday and that emergency teams were immediately dispatched to the scene.

“During times like this, US Steel employees come together to extend their love, prayers, and support to everyone affected,” David Burritt, CEO of the company, said in a statement.

Some US media outlets reported that people were still trapped under the rubble of the explosion.

Videos on social media, not verified by AFP, appeared to show firefighters battling the blaze in front of a gutted industrial building, under a thick plume of white smoke.

The Clairton Coke Works is the largest coking factory in the United States – a facility where coal is processed to produce coke, a key fuel in steelmaking.