Union and oil company negotiators have been meeting since January 17 to hammer out a new three-year agreement for workers at nearly two-thirds of U.S. refining capacity.

Union negotiators have not sent out a similar warning to refinery workers in the past three rounds of contract talks with the industry.

Shell Oil Co, which is the lead oil company negotiator, said on Saturday it would work toward an agreement.

"We continue to negotiate with the goal of reaching a mutually satisfactory agreement with USW," said Shell spokeswoman Kayla Macke.

Previously, Shell had said it was optimistic a deal could be reached.

In the statement issued Saturday night, the USW's bargaining committee said that without further progress in the talks a strike could take place at "one or more locations when the collective bargaining agreements expire."

Talks between the two sides were scheduled to continue through the weekend.

USW spokeswoman Lynne Hancock said the union would make no further statements on Saturday.

In the lead-up to this year's talks, the Steelworkers union warned that the lack of improved safety protections for workers at the nation's refineries could bring about the first nationwide strike since 1980.

Eighteen workers have died at U.S. refineries since the last agreement in 2009, according to the union.

As much as 11 percent of U.S. refining capacity could temporarily shut due to a strike lasting three months, sources have said.

In addition to possible shutdowns, oil companies have been training temporary replacement workers to continue to operate their refineries in the event of work stoppage.

One source said refiners' preparations for a possible strike are unlike any seen in 20 years.

Industry analysts have said they thought the chances of a strike were slim.

During the 1980 strike, refiners kept their plants in operation by using trained supervisors and engineers to replace hourly workers who operate and maintain equipment.

Shell Oil Co is the U.S. unit of energy giant Royal Dutch Shell Plc (RDSa.L).

(Editing by Chistopher Wilson)

• A Permanent Economic Emergency