Flames and heavy smoke billow out of the plant owned by US Steel as firefighters struggle to extinguish the fire.

Multiple explosions at a US Steel plant near Pittsburgh, Pennsylvania have killed two people and injured 10, according to the company and local authorities.

The blasts at the Clairton Coke Works – part of a sprawling industrial complex along the Monongahela River – took place just before 11am Eastern Time (15:00 GMT) on Monday.

Firefighters battled flames and heavy smoke that billowed out of the plant, which is owned by US Steel, a subsidiary of Nippon Steel.

Initially, two people were reported missing. One person was found and transported to a local hospital, said Allegheny County Police Assistant Superintendent Victor Joseph at an afternoon briefing.

There was no word yet on the possible cause of the explosion.

The investigation into the explosion would be “a time-consuming technical investigation”, Joseph said.

David Burritt, president and chief executive officer of US Steel, said in a statement that the company was working with local authorities to discover the cause.

Pennsylvania Governor Josh Shapiro posted on X that there were multiple explosions at the plant and that his administration was in touch with local officials.

“The scene is still active, and folks nearby should follow the direction of local authorities,” he wrote at the time the employee was missing.

The severity of the injuries was not known, but news accounts said several people were taken to hospital burn units.

Steel sector in decline

Clairton Mayor Rich Lattanzi said it was a horrible day for the city, about 32km (20 miles) south of Pittsburgh, long known as the US Steel City.

US Steel has produced steel in the area since the late 19th century, but in recent decades, the industry has been in decline, leading to plant closures and restructurings.

In June, Nippon Steel, Japan’s biggest steelmaker, closed its $14.9bn acquisition of US Steel after an 18-month struggle to obtain United States government approval for the deal, which faced scrutiny due to national security concerns.

While air quality monitors did not detect a dangerous rise in sulphur dioxide after Monday’s explosions, residents within 1.6km (1 mile) of the plant were advised to remain indoors, close windows and doors, set HVAC systems to recirculate, and avoid activities that draw in outside air, said Allegheny County Executive Sara Innamorato at the briefing.

The Clairton Coke Works is the largest coke manufacturing facility in the US, employing about 1,300 workers. It operates 10 coke oven batteries, which produce about 4.3 million tonnes of coke a year.

Coke is produced by heating coal at high temperatures. It is used in blast furnaces as part of the process of making steel.