A union representing Starbucks workers said Thursday that baristas in Los Angeles, Chicago and Seattle would walk off the job Friday morning and that the strikes would spread to hundreds of stores by Christmas Eve unless the company improved its wage offer in contract negotiations.

The union, which represents baristas at more than 500 company-owned stores in the United States — about five percent of the U.S. total — said it called the strike after a bargaining session with the company this week failed to produce better wage gains.

The strike is expected to begin in about 15 stores across the three metropolitan areas, according to a union member familiar with the situation who was not authorized to speak publicly.

“Starbucks proposed an economic package with no new wage increases for union baristas now and a guarantee of only 1.5 percent in future years,” the union, Workers United, said in a statement.

The guarantee would entitle unionized Starbucks workers to receive a wage increase of 1.5 percent even if the company raises wages nationwide by less than that amount in future years. If the company raised wages by more than that — as it did this year, with a recently announced increase of 2 percent — unionized workers would get the higher amount.

Andrew Trull, a Starbucks spokesman, said union delegates “prematurely ended” this weeks’ negotiations. “It is disappointing they didn’t return to the table given the progress we’ve made to date,” he added.