The strike at U.S. refineries has been widened and now includes plants accounting for one-fifth of national production capacity, union officials said at the weekend.

Workers at 12 refineries and three other facilities have now walked off the job, in the biggest refinery strike in the United States for 35 years.

The United Steelworkers union (USW) delivered strike notices to four more plants on Saturday.

A work stoppage was expected to start midnight Saturday at the Motiva Enterprises refinery in Port Arthur, Texas. This refinery, the nation's largest, produces more than 600,000 barrels per day (bpd). It is operated by Motiva Enterprises LLC, a 50-50 joint venture between Shell Oil Company (U.S. subsidiary of Royal Dutch Shell) and Saudi Refining, Inc. (a subsidiary of Saudi Aramco).

Additionally, 24-hour strike notices were delivered at the joint venture's two Louisiana refineries in Convent and Norco, which produce 235,000 and 238,000 bpd respectively, as well as at the Shell Chemical plant in Norco.

As many as 6,550 USW members employed at the 15 plants are thought to be involved in the work stoppage, which began on February 1 when negotiations for a new three-year labor contract broke down.

Commenting on the union's latest move, USW International President Leo W. Gerard said: "The industry's refusal to meaningfully address safety issues through good faith bargaining gave us no other option but to expand our work stoppage."

USW Vice President Tom Conway added: "We're committed to reaching a settlement that works for both parties but adequate staffing levels, worker fatigue and other important safety issues must be addressed."

The USW represents employees at more than 200 refineries, oil terminals, pipelines and petrochemical plants across the United States.