Union negotiators on Thursday rejected the latest contract offer from oil companies and said the largest U.S. refinery strike since 1980 may spread to more plants beyond the 11 where walkouts are underway.

The United Steelworkers union (USW) said in a message to members and news media that the latest proposal from lead oil company negotiator Royal Dutch Shell Plc failed to improve safety at refineries and chemical plants in an "enforceable way."

The union also told workers not on strike to be prepared to walk out in the coming days.

Reporting by Erwin Seba