(Reuters) -More than 1,000 Starbucks ( SBUX ) unionized baristas in over 40 U.S. cities launched an open-ended strike on Thursday, the union said, escalating their push for a collective bargaining agreement over pay and other benefits at the coffee giant.

The walkout, which the union said will begin with 65 stores, coincides with Starbucks' ( SBUX ) Red Cup Day, a busy holiday sales event that typically drives higher customer visits.

Starbucks ( SBUX ) did not immediately respond to a Reuters request for comment.

The Starbucks Workers United, representing employees at more than 550 stores in the United States, said it plans rallies at 4 p.m. local time in more than a dozen cities and warned the strike could become the largest and longest in Starbucks' ( SBUX ) history.

Stores in cities, including Seattle, New York, Philadelphia, Dallas, Austin and Portland, will join the work stoppage, the union said.

Last week, unionized baristas voted to authorize a strike if a contract was not finalized by November 13.

Each side has blamed the other for ending talks late last year, and has said they are ready to return to negotiations.

Starbucks ( SBUX ) says the union represents roughly 9,500 workers, or 4% of its cafe workforce.