A total of 6,550 USW members are on strike at 15 plants, including 12 refineries accounting for one-fifth of U.S. capacity. Union members work at more than 200 oil terminals, pipelines, refineries and chemical plants in the U.S.
The USW has said it is seeking to retain safety provisions from previous contracts and tighten fatigue standards for workers, as well as win back daily maintenance jobs now done by non-union contractors.
"(The strike spreading) depends on what happens in the next round of negotiations and that those negotiations resume fairly quickly," Gerard in a telephone news conference from Atlanta.
Gerard, who is attending the AFL-CIO winter conference in Atlanta, said no date has been set for resuming negotiations.
A Shell spokesman also said a resumption of talks had not been scheduled as of Tuesday morning. Shell Oil Co, the U.S. arm of Royal Dutch Shell Plc [RDSa.L], is the lead oil company negotiator.
Talks broke off on Friday, after which the USW ordered workers at three Motiva refineries, including the nation's largest, which are co-owned by Shell, to walk off their jobs on Saturday and Sunday.
Sources familiar with the talks told Reuters on Monday that face-to-face negotiations may not resume this week. That was a change from the weekend, when sources said meetings might resume by the middle of this week.
Gerard also said a "handful" of USW members have crossed picket lines to return to their jobs at refineries and other plants.
Tesoro Corp, which owns three West Coast refineries where workers are on strike, said some workers have returned to their jobs at all three plants. It declined to say exactly how many workers have returned to their jobs.
Gerard said fires or explosions have occurred every eight days on average since an April 2010 blast at Tesoro's refinery in Anacortes, Washington, which claimed the lives of seven workers.
"From 2010 to now, there have been 27 people who have been killed," Gerard said, referring to deaths from accidents at refineries.