Multiple explosions at a US Steel plant near Pittsburgh have killed at least one person and injured 10 others, authorities said on Monday, and emergency crews were searching through the rubble for an employee believed to be missing.

The blasts at the Clairton Coke Works – part of a sprawling industrial complex along the Monongahela River –took place just before 11am local time.

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.

The other individual remains missing, he said.

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

"It's still a rescue mission," Joseph said, adding that the probe into the explosion would be "a time-consuming technical investigation."

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.

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

Clairton Mayor Rich Lattanzi said it was a horrible day for the city, located about 32km south of Pittsburgh, long known as America's 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 US$14.9 billion acquisition of US Steel after an 18-month struggle to obtain US government approval for the deal, which faced scrutiny due to national security concerns.

The Clairton Coke Works is the largest coke manufacturing facility in the United States, employing about 1,300 workers.

It operates 10 coke oven batteries, which produce about 4.3 million tons 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. (Reuters)