A 20-day-old nationwide strike at U.S. oil refineries escalated Saturday when workers at the country’s largest refinery joined the picket line as the United Steelworkers union (USW) pushed for higher wages and improved workplace safety conditions, Reuters reported.  The USW said Saturday the strike at the Motiva Enterprises refinery in Port Arthur, Texas, would begin at midnight.

“In addition, 24-hour strike notices were delivered at Motiva’s two Louisiana refineries in Convent and Norco as well as at the Shell Chemical plant in Norco,” the USW said in a prepared statement. The union represents 850,000 workers in various industries. About 1,350 USW workers are employed at the three refineries and chemical plant.

Work stoppages have hit a dozen oil refineries that represent a fifth of the country’s crude processing capacity in the first nationwide strike to hit refiners in 35 years. Affected plants have been kept running with temporary workers.

The Motiva plant is a 50-50 joint venture between the U.S. subsidiaries of Royal Dutch Shell and Saudi Aramco, the Saudi state-owned oil giant. The stoppages announced Saturday process more than a million barrels of oil a day. 

The nationwide strike began Feb. 1. The last time the strike expanded was Feb. 6 when BP workers walked off the job in Ohio and Indiana after the USW said industry negotiators are guilty of “bad-faith bargaining” in talks over improved wages, benefits and safety procedures.

Shell, which is negotiating the pattern contract, said earlier Saturday negotiations Friday failed to yield an agreement.

“BP remains at the negotiating table and is committed to reaching an agreement that provides good wages while giving management the flexibility it needs to enhance safety, improve efficiency and remain competitive with others in the industry,” BP spokesman Scott Dean said in a statement earlier this month, according to FuelFix. “We hope this strike will be as short as possible and for workers to return with a contract that ensures long-term prosperity for everyone.”