AT LEAST one person has died and dozens are injured after a massive explosion ripped through a steel plant in Pennsylvania on Monday.

An urgent rescue operation is underway to save workers who were left trapped under the piles of rubble left by the fiery boom.

Fire crews raced to the scene to battle the flames after the explosion at US Clairton Coke Works plant right outside Pittsburgh.

Many people were injured in the incident, at least one person has died, and two people are unaccounted for, according to ABC affiliate WTAE.

Terrifying aerial images show the piles of singed wreckage left by the explosion.

First responders say that workers are still trapped under the mountains of heavy materials, and that fire crews are trying to pull them out.

The explosion has been declared a mass casualty incident so that Clairton first responders can get additional EMS resources.

Multiple medical helicopters were called to assist rescue efforts.

The Allegheny County Health Department has recommended that individuals residing within a one-mile perimeter of the facility remain inside, ensuring all doors and windows are securely closed.

HVAC systems should also be set to recirculate, and exhaust fans shouldn’t be used, officials said.

Dangerous levels of PM2.5, small particles that can get trapped in the lungs, and SO2, sulfur dioxide, have not yet been detected in the air.

Nearby hospitals are treating at least 14 patients, and their conditions remain unknown, NBC affiliate WPXI reported.

Pennsylvania Governor Josh Shapiro wrote that he is in touch with local officials as they continue to work through the scene.

“The scene is still active, and folks nearby should follow the direction of local authorities,” he said on X.

“Please join Lori and me in praying for the Clairton community.”

Lieutenant Governor Austin Davis also said that the state would be sending “whatever resources and manpower are needed” for the rescue.

‘FELT LIKE THUNDER’

A local who was working near the explosion described the terrifying moment he felt the ground-shaking boom.

“Felt like thunder,” Zachary Buday told WTAE.

“It shook my chest. It shook the building, then we saw the dark smoke rising from the steel mill.

“Put two and two together. It’s like something bad happened.”

Clairton Coke Works is the largest coking operation in North America, according to the Associated Press.

Coking is the process of heating coal past 1,000 degrees to refine it into a more high-carbon product called coke.

The sprawling facility employs thousands of workers and is one of four major operations in Pennsylvania.