(Reuters) - A strike at Starbucks' ( SBUX ) U.S. stores will expand to over 300 stores on Tuesday, with more than 5,000 workers expected to walk off the job, before the five-day work stoppage ends later on Christmas Eve, the workers' union said.

Starbucks Workers United, representing employees at 525 stores nationwide, said more than 60 U.S. stores across 12 major cities, including New York, Los Angeles, Boston and Seattle, were shut on Monday.

Talks between Starbucks ( SBUX ) and the union had hit an impasse with unresolved issues over wages, staffing and schedules, leading to the strike.

The Christmas Eve strike on Tuesday was projected to be the largest ever at the coffee chain, the union added.

Starbucks ( SBUX ) did not immediately respond to a Reuters request for comment outside regular business hours.

"These strikes are an initial show of strength, and we're just getting started," an Oregon barista said in a union statement.

The Seattle-headquartered firm has previously said it was ready to continue negotiations, claiming the union delegates prematurely ended the bargaining session.

Earlier this month, the workers' group rejected an offer of no immediate wage hike and a guarantee of a 1.5% pay increase in future years.

The union also said that Starbucks ( SBUX ) has yet to present its workers with "a serious economic proposal."

(Reporting by Bipasha Dey, Shubham Kalia and Gursimran Kaur in Bengaluru; Editing by Abinaya Vijayaraghavan)