A strike at Starbucks will expand to over 300 U.S. 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 and the union had hit an impasse with unresolved issues over wages, staffing and schedules, leading to the strike that began on Friday.With a union contract, improved benefits, wages, and working conditions could be in writing and cant be reduced without bargaining, according to the Workers United website.The Christmas Eve strike on Tuesday was projected to be the largest ever at the coffee chain, the union added. These strikes are an initial show of strength, and were just getting started, an Oregon barista said in a union statement.When asked for a response, a Starbucks spokesperson referred to a company statement it released on Monday.It said that 97% to 99% of Starbucks stores will continue to operate and serve customers, adding that it expects a very limited impact to overall operations. Starbucks has over 10,000 company-operated stores across the U.S.We are ready to continue negotiations when the union comes back to the bargaining table, the company said.The Seattle-headquartered firm had previously claimed that the union delegates prematurely ended the bargaining session.Earlier this month, the workers group rejected an offer of no immediate wage raises and a guarantee of a 1.5% pay increase in future years.The union also said that Starbucks has yet to present its workers with a serious economic proposal.Bipasha Dey, Shubham Kalia, Gursimran Kaur and Savyata Mishra, Reuters