Workers at Starbucks will walk off the job Friday in three US cities in a strike their union threatened could spread around the country in the busy run-up to Christmas.
The announcement, which will initially affect stores in Los Angeles, Chicago and the firm's home city of Seattle, comes as online giant Amazon was also hit by a walkout in the crucial final shopping days of the festive period.
Starbucks Workers United, which says it represents baristas at hundreds of outlets around the country, said its action was aimed at forcing the company to improve pay and conditions after months of negotiations that it said have gone nowhere.
"Nobody wants to strike. It's a last resort, but Starbucks has broken its promise to thousands of baristas and left us with no choice," a union press release quoted Texas barista Fatemeh Alhadjaboodi as saying.
The strike, which the union says will hit more outlets every day until Tuesday, comes as Starbucks grapples with stagnating sales in key markets.
Former Chipotle boss Brian Niccol was brought on board this year with a mandate to staunch a decline that saw quarterly revenue worldwide fall three percent to $9 billion.
"In September, Brian Niccol became CEO with a compensation package worth at least $113 million," thousands of times the wage of the average barista, said union member Michelle Eisen in the statement.
The union said Starbucks had not engaged fruitfully for several months, and threatened it was ready to "show the company the consequences."
"We refuse to accept zero immediate investment in baristas' wages and no resolution of the hundreds of outstanding unfair labor practices," said Lynne Fox, president of Workers United.
"Union baristas know their value, and they're not going to accept a proposal that doesn't treat them as true partners."
Starbucks did not immediately respond to an AFP request for comment.
