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SANTIAGO (Reuters) - As many as 17,000 workers at Walmart´s largest union in Chile walked off the job early Wednesday morning, demanding better salaries after negotiations over a new labor contract broke down earlier this week, the union’s president said.

The strike was expected to shut down more than one-quarter of the approximately 400 stores that Walmart Inc WMT.N operates in Chile, some of them under the popular grocery store franchise Lider.

“We haven´t reached an agreement. We did make some advances, but they weren´t enough to seal a deal of the magnitude the company has in mind,” Juan Moreno, president of Lider´s Inter-Company Union (SIL), said in a statement.

Union leaders said the key sticking point was a salary increase. As automation in stores has increasingly replaced workers, unionized workers demanded the company boost salaries by 4 percent. Walmart Chile has offered 3 percent, Moreno said.

The U.S. retail giant said in a statement late Wednesday night that it had offered a “robust” contract offer that addressed key points, including the salary increase, though it did not specify how much the company had offered to raise pay.

The strike would primarily affect stores under the franchise SuperBodega aCuenta, and, to a lesser extent, Express, Lider and Ekono, according to the statement.

The company said it would begin informing customers about alternative shopping venues on Wednesday amid the shutdown.