Starbucks (SBUX) filed Unfair Labor Practice charges against the union backing baristas, an escalation of tensions between the coffee giant and pro-union employees.

In two complaints filed with the National Labor Relations Board (NLRB), the coffee chain alleged that the Workers United labor organization exhibited behavior in Phoenix and Denver that was "reasonably expected to physically intimidate and bully partners and customers in retaliation for their withholding support of Workers United."

Starbucks Workers United, the subsidiary representing baristas, did not respond to a request for comment.

Workers United has helped Starbucks partners, as employees are known, file over 90 Unfair Labor Practice charges against the Seattle-based company amid efforts to unionize. The new complaints are the first time that Starbucks has officially made Unfair Labor Practice charges against Workers United.

In a letter sent to U.S. staff and obtained by Yahoo Finance, Starbucks Executive Vice President Rossann Williams stated that allegations include "blocking entrances and exits, making threats, yelling profanities, and surrounding a store and pounding on the windows to physically intimidate and bully partners inside in retaliation for not supporting Workers' United organizing drive."

In a phone interview, Starbucks Senior Vice President of Global Communications and Publica Affairs AJ Jones told Yahoo Finance that the some partners asked the company to intervene after becoming "very much concerned about the Workers United organizers threatening their livelihood and their physical well being."

One of the locations listed in the Starbucks complaints is 7000 East Mayo Blvd in Phoenix, which is the source of a previous labor complaint brought on behalf pro-union employees over alleged union busting activity. A second complaint against Starbucks related to the same location was recently filed, and a hearing before a NLRB administrative law judge is set for June.