(Reuters) - The United Auto Workers union (UAW) has filed a complaint accusing Tesla Inc Chief Executive Elon Musk of illegally threatening to take away benefits from workers who join the union.

FILE PHOTO: Elon Musk listens at a press conference following the first launch of a SpaceX Falcon Heavy rocket at the Kennedy Space Center in Cape Canaveral, Florida, U.S., February 6, 2018. REUTERS/Joe Skipper/File Photo

The UAW, which is seeking to represent workers at Tesla’s facility in Fremont, California, filed the complaint with the National Labor Relations Board late on Wednesday.

Musk in a tweet on Monday said there was nothing stopping Tesla workers from joining a union, but “why pay union dues & give up stock options for nothing?”

The union says Musk violated the National Labor Relations Act, which prohibits employers from making threats or promises to workers to discourage them from joining unions.

Tesla in a statement said Musk’s comment simply recognized that other automakers whose workers are represented by the UAW do not provide stock options.

“UAW organizers have consistently dismissed the value of Tesla equity as part of our compensation package,” the company said.

In a separate tweet, Musk accused the UAW of driving General Motors and Chrysler to bankruptcy and losing “200,000+ jobs for people they were supposed to protect.”

He was apparently referring to effects of the crisis in the U.S. auto industry in 2008-2010.

UAW President Dennis Williams during a press briefing in Detroit on Thursday called Musk’s comments “ridiculous.”

“I don’t know what the hell Musk is up to,” Williams said. “Sometimes I scratch my head.”

The new complaint is the latest salvo in an ongoing legal battle between Tesla and the UAW, which has accused the company of utilizing a series of unlawful tactics to thwart unionizing.

In August, based on a separate UAW complaint, the NLRB found that Tesla had violated workers’ rights by requiring them to sign a confidentiality agreement that could bar them from talking about their working conditions and safety issues at the California plant.

In October, the UAW filed yet another complaint with the NLRB accusing Tesla of firing 400 workers who supported the union’s campaign. The electric carmaker is also facing lawsuits accusing it of discriminating against black, gay, and older workers. Tesla has denied those claims.

Williams told reporters that a decision on when to hold a unionization vote at the Tesla plant will not come until a new UAW president is seated later this year.

A challenge for the union and its organizers has been the high rate of worker turnover at Tesla.