A Starbucks Corp. strike of unionized baristas has spread to three more cities in the US, disrupting service during the final days of the crucial holiday season.

Baristas from Denver, Pittsburgh and Columbus, Ohio, will join strikes that began on Friday in Chicago, Los Angeles and Seattle, according to a union statement.

Starbucks employees from additional cities will join each day until Christmas Eve, unless the company honors its February commitment to resolve unfair labor practices, the union added.

The disruptions have had "no significant impact" to store operations and only "a small handful" of US stores have been affected, ...