Starbucks baristas at locations in a dozen states have walked off the job as part of a holiday strike they say could spread to hundreds of stores by Tuesday unless the company ups its wage offer in contract negotiations that have been ongoing for the better part of the year.

Earlier, Starbucks said the union “prematurely ended” a bargaining session and called for talks to continue. The coffee chain has said it’s focused on improving workers’ experience and that baristas who work at least 20 hours a week receive $30 an hour, on average, when combining pay and benefits.