CLAIRTON, Pa. (AP) — An oven at a U.S. Steel plant near Pittsburgh exploded Wednesday, injuring 21 workers, at least three critically, causing a fire that burned for hours, emergency officials said.

The powerful blast in the coke oven at United States Steel Corp.’s Clairton Coke Works happened around 9:30 a.m., Allegheny County spokesman Kevin Evanto said. Most of the workers suffered burns; one suffered chest pains.

‘‘It’s a miracle that anybody even walked away from that,’’ Allegheny County Emergency Services Chief Bob Full told reporters at the scene. He said the explosion was so mighty it bent steel beams and destroyed block walls.

Everyone had been accounted for, and the cause of the explosion was being investigated, union and company officials said. The Occupational Safety and Health Administration had a team of investigators on site, spokeswoman Leni Fortson said.

‘‘It was a big boom and then everything just went black,’’ janitor John Chappell, 59, of Clairton, told the Pittsburgh Tribune-Review as he left UPMC Mercy. He was not injured.

‘‘It was pitch black but you could tell there was debris flying all over the place,’’ Chappell told the newspaper. ‘‘I’m just blessed because I know it could have been worse.’’

An air quality inspector at the plant at the time of the blast said he saw a large cloud of smoke that dissipated quickly, said Jim Thompson, manager of the Allegheny County Air Quality Program. Thompson said that and other factors indicate the explosion may have been caused by the gas used to heat one of the coke ovens.

Neighbors said they heard alarms at the plant but didn’t know at first whether it was a real emergency.

‘‘They always play their siren,’’ said Tiarra Williams, 17, who lives on a hill overlooking the plant. A maintenance worker died in a September 2009 explosion at the plant, which sits in a valley along the Monongahela River about 20 miles south of Pittsburgh.

About 1,500 people work at the plant, said Michael Wright, head of the health, safety and environment department for the United Steelworkers union. The company said 15 employees and six contractors were injured.

Coke, a raw material used in steelmaking, is coal that is baked for a long time at a high temperature to remove impurities. The coal is baked in special ovens, several of which make up a coal battery; there are 12 batteries at the Clairton plant.

The explosion happened during maintenance in the B battery, which consists of 75 ovens. The battery is located on the northern side of the sprawling plant and was shut down after the explosion, but a U.S. Steel spokeswoman said the rest of the plant was operating normally.

U.S. Steel calls its Clairton plant the biggest coke manufacturing facility in the U.S., producing about 4.7 million tons per year.

At Pittsburgh’s West Penn Hospital, two workers in their 50s were in critical condition with chemical burns in their airways as well as burns to their heads, necks and faces, said Dr. Larry Jones, the hospital’s director of emergency medicine.

‘‘The burns themselves are serious burns, but with the inhalation injury on top of it, these are very, very serious, a very serious situation,’’ Jones said.

A third worker, in his 40s, was in serious condition with burns on his head, neck, face and hands, and an ankle fracture, Jones said.

Six workers, men ranging in age from 20 to 50, were taken to UPMC Mercy, said Dr. Alain Corcos, medical director of UPMC Mercy’s trauma and burn centers. One of those was in critical condition with airway burns.

‘‘They are all expected to recover,’’ Corcos said.

One injured worker was taken to UPMC McKeesport, and three were taken to UPMC Presbyterian, she said.

One person was admitted to Jefferson Regional Medical Center in Jefferson Hills with chest pains, spokeswoman Candy Williams said. Three other workers were treated and released for injuries including burns and inhaling dust, she said.

The company said four other workers were treated at the plant’s medical facility and released.

William Magyar, 44, of McKeesport, was cleaning a rental property near the plant when he said he heard alarms around 10 a.m.

‘‘I figured it was a fire I didn’t smell,’’ Magyar said.

Elaine Lawrence, 53, whose son works at the plant, was lying on the couch at her Clairton home when her daughter told her there had been an explosion. The daughter drove her to the plant, but they weren’t allowed in; they headed to a hospital, and that’s when her son called.

‘‘He said he had just passed where the explosion happened to go to the other block and suddenly he heard an explosion,’’ Lawrence said. Martin Lawrence, 19, was not injured and remained at work, she said.

‘‘I was real concerned, because that’s my only son,’’ she said.