Bloomberg News/file 2007Union workers have struck at Motiva’s Port Arthur, Texas, refinery, the largest in the U.S. with a 660,000-barrel-per-day capacity.A nearly monthlong strike at U.S. refineries escalated early Saturday morning when workers at the nation’s largest refinery in Texas walked off the job after another round of negotiations failed. Union leaders warned that workers at other large refineries in Louisiana would also strike by Sunday if the dispute doesn’t end. Refineries have largely continued operating amid the labor strife, but plants accounting for roughly 16.5% of U.S. capacity are now affected, up from 13% before. If workers walk off at the additional plants on Sunday, the strike will affect refineries representing 19% of U.S. capacity. United Steelworkers and Royal Dutch Shell RDS.A, -0.96% which leads bargaining efforts for the refining industry, have been locked in a stalemate for weeks as they attempt to hammer out a new three-year contract that will set the pattern on wages, benefits, and safety issues for individual refineries. The union has so far rejected seven offers from Shell, and its members have been striking at some plants since the beginning of the month, when talks broke down in the final hours of bargaining before a Feb. 1 deadline. After the latest negotiations ended late Friday evening, workers at the Motiva Port Arthur Refinery in Texas, the country’s largest refinery with a capacity of over 600,000 barrels a day, walked off the job shortly after midnight. Motiva Enterprise LLC is co-owned by Shell and a subsidiary of Saudi Aramco. “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 International President Leo W. Gerard said in a statement. Union leaders also warned that workers at several additional refineries in Louisiana were prepared to strike starting Sunday. Shell confirmed that it received strike notices for the Motiva Convent, Motiva Norco and Shell Chemicals Norco plants. The union’s initial walk out earlier this month included nine sites. If union workers leave their jobs as planned in Louisiana, more than 6,500 USW members employed at 15 facilities, including refineries and other sites, will be on strike, up from 5,000 previously. An expanded version of this report appears at WSJ.com