More than 1,000 Starbucks unionized baristas in over 40 U.S. cities launched an open-ended strike 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' Red Cup Day, a busy holiday sales event that typically drives higher customer visits.

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