The workers’ union representing more than 10,000 baristas at Starbucks said its members will strike at stores in Los Angeles, Chicago, and Seattle on Friday morning, citing unresolved issues over wages, staffing and schedules.

Workers United, representing employees at 525 Starbucks stores across the U.S., said on Thursday that walkouts will escalate daily, and could potentially reach hundreds of stores nationwide by Dec. 24, unless an agreement is reached.

The union, which has been urging Seattle-headquartered Starbucks to increase wages and staffing at its stores as well as implement better schedules, announced a five-day strike that could impact holiday sales.

Starbucks said it is ready to continue negotiations, claiming the union delegates prematurely ended the bargaining session this week.

The union and Starbucks established a “framework” in February to guide organizing and collective bargaining. Negotiations between the company and Workers United began in April, based on the framework, which could also help resolve numerous pending legal disputes.

The company said it has held more than nine bargaining sessions with Workers United since April, and reached more than 30 agreements on “hundreds of topics”, including economic issues.

“Since the February commitment, the company repeatedly pledged publicly that it intended to reach contracts by the end of the year, but it has yet to present workers with a serious economic proposal,” the union said.

The coffee chain is undergoing a turnaround under its newly appointed top boss, Brian Niccol, who aims to restore “coffee house culture” by overhauling cafes, adding more comfortable seating, reducing customer wait times to less than four minutes, and simplifying its menu.

Labor actions have picked up pace across service industries. Amazon.com workers at seven U.S. facilities walked off the job early on Thursday during the holiday shopping rush.