FILE PHOTO: A Tesla charging station is seen in Salt Lake City, Utah, U.S. September 28, 2017. REUTERS/Lucy Nicholson

(Reuters) - The United Auto Workers (UAW) International Union said on Thursday it filed a complaint against electric carmaker Tesla Inc TSLA.O on behalf of the company's terminated workers.

The unfair labor practice (ULP) charges were filed at the National Labor Relations Board’s (NLRB) Oakland office, the union said.

Tesla in October fired about 400 employees including associates, team leaders and supervisors, Reuters reported, citing a former employee.

Performance reviews can result in promotions and occasionally in employee departures and no action was taken based on their feelings on unionization, Tesla said in an email to Reuters on Thursday.

Roughly 20,000 ULPs are filed with the NLRB by unions like the UAW as an organizing tactic, the company said.

UAW said in February it will greet Tesla’s Fremont, California, assembly plant workers with ‘open arms’ in a bid to unionize the factory.

NLRB, the U.S. agency in charge of enforcing labor law, in late August filed a complaint against Tesla, saying it found merit to workers’ complaints about unfair labor practices.