PITTSBURGH — Health officials have ordered U.S. Steel to stop its violation of federal sulfur dioxide standards at three Pittsburgh-area processing plants following a Christmas Eve fire that affected pollution control at one of its plants.

The Allegheny County Health Department issued the enforcement order Thursday. It says U.S. Steel must reduce its use of coke oven gas and daily sulfur dioxide emissions at Mon Valley Works. U.S. Steel spokeswoman Meghan Cox said the plant is working around the clock to resolve the issue.