Union leaders called strikes on Sunday at nine U.S. refineries in a bid to pressure oil companies to agree to a new national contract covering workers at 63 plants.

The walkouts, the first held in support of a nationwide pact since 1980, target plants that together account for more than 10 percent of U.S. refining capacity. The discord comes as plunging crude prices force oil companies to slash spending.

Royal Dutch Shell, the lead industry negotiator, indicated talks had Ubroken down.

"We remain committed to resolving our differences with USW at the negotiating table and hope to resume negotiations as early as possible," Shell said.

Shell activated a strike contingency plan at its sprawling joint venture refinery in Deer Park, Texas, to keep operating normally.

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Other companies have said they could call on trained managers to use as replacement workers, so the strikes are not expected to cause gasoline prices to spike.

The United Steelworkers union (USW) did not say at which plants the strikes would be held.

But one source familiar with union plans said walkouts were called at Exxon Mobil Corp's refinery in Beaumont, Texas, two Marathon Petroleum refineries and LyondellBasell's plant near Houston.

Also included were Marathon's refinery in Kentucky, Tesoro Corp's Benicia and Martinez plants in California, and its refinery in Anacortes, Washington, the source said.