The four BP employees were taken to local hospitals as a result of the incident, the company said. No information was immediately available as to how they were injured or about their condition.

Work on an electrical substation led BP to reduce production on two of three crude distillation units at the refinery on Thursday.

The Whiting refinery, located in the Chicago suburbs, is BP's largest in the United States.

(Reporting by Erwin Seba, editing by G Crosse)