Workers at three major US oil refineries operated with Royal Dutch Shell went on strike Saturday in a dispute over safer working conditions, the United Steelworkers (USW) union confirmed.
Around 1,350 employees in three refineries and chemical plants are affected by the latest strike action, which mirrored walkouts on other sites launched earlier this month.
Some 5,200 workers spread across around a dozen installations have been on strike since February 1 demanding improved wages and safety conditions.
The USW said the strike extension involved workers at the massive Motiva Enterprises refinery in Port Arthur, Texas, the largest refinery in the United States which produces more than 600,000 barrels per day.
The plant is a 50-50 joint enterprise between Shell Oil Company and Saudi Refining, a subsidiary of Saudi Aramco.
The USW said it had also delivered strike notices at two Motiva refineries in Louisiana and at the Shell Chemical plant in Norco. The refineries are also jointly operated by Royal Dutch Shell and Saudi Refining.
Around 1,350 USW workers are employed at the three refineries and the chemical plant, which have a total daily capacity of 1,073,260 barrels per day.
"We're committed to reaching a settlement that works for both parties," USW vice president Tom Conway said in a statement.
"But adequate staffing levels, worker fatigue and other important safety issues must be addressed."