U.S. labor officials are petitioning a federal court to force Starbucks to bring back activist employees who they say were removed for their union campaigning, according to a Friday filing.

The National Labor Relations Board’s Phoenix chief is seeking an injunction against Starbucks that would require it to reinstate three employees that were allegedly illegally discharged, forced out or placed on unpaid leave.

The filing marks the latest in what’s expected to be a lengthy and expensive legal battle between a union campaign and the global coffee chain.

Since August, more than 200 Starbucks locations have filed paperwork to unionize under Workers United, an affiliate of the Service Employees International Union. So far, 24 stores have voted to unionize, with only two locations so far voting against.

But tension has escalated between the two sides, with each accusing the other of lawbreaking activity. Workers United has filed dozens of complaints of its own with the NLRB against Starbucks, alleging that the company has illegally retaliated against, harassed and fired organizers in cafes across the country. The government agency has also issued complaints against Starbucks.

For its part, Starbucks this week filed two complaints with the NLRB, alleging that the union organizing its baristas broke federal labor law. Starbucks did not immediately respond to a request for comment.

Friday’s filing argues that Starbucks retaliated against the three employees after learning its employees were involved in pro-union activity. “Immediate injunctive relief is necessary to ensure that the Employer does not profit nationwide from its illegal conduct,” NLRB Director Cornele Overstreet said in a statement.

— CNBC’s Amelia Lucas contributed to this report.