US Steel said the incident happened around 11 am (1500 GMT) on Tuesday and that emergency teams were immediately dispatched to the scene.
“During times like this, U.S. 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 had reported that people were 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.
– Agence France-Presse