Four people were injured when a fire erupted on Dec. 23 at ExxonMobil's complex in Baytown, Texas, one of the largest refining and petrochemical facilities in the U.S.

Those injured were in a stable condition and all other personnel were accounted for, Exxon said.

An Exxon official told a news conference the blaze had affected a unit that produces gasoline.

The Baytown plant houses a chemical plant, an olefins plant and the country's fourth biggest oil refinery, with capacity to process 560,500 barrels per day of crude.

The fire occurred in a hydrotreater unit at the oil refinery that was shut on Dec. 22 due to a bypass line leak, sources familiar with plant operations said, adding that those injured were contractors who had been repairing the leak.

Three of the injured were flown to hospital by Lifeflight rescue helicopter and a fourth person was taken by ambulance, Harris County Sheriff Ed Gonzalez said on Twitter.

He said initial reports had indicated there had been some type of explosion at the plant. Social media users said on Twitter that a blast shook buildings in the area.

The Baytown olefins facility, which began operations in 1979, is one of the largest ethylene plants in the world, according to the company's website.

Production was reduced across the Baytown refining and petrochemical complex in August 2019 because of a fire in a propylene recovery unit at the olefins plant.

As many as 37 workers were injured in another fire at the olefins facility in July 2019.