The US agency charged with enforcing labor laws filed a complaint on Thursday against Tesla, saying it found merit in worker accusations of labor rights infringement.

The board said Tesla had violated rights of workers by requiring them to sign overly-broad confidentiality agreements that could bar speaking out over labor conditions and safety issues at the Fremont, California facility.

The NLRB also said it was investigating Tesla for allegedly intimidating and harassing workers, charges the car maker denies.

"In recent months, workers have raised concerns about health and safety, compensation, and their right to speak out," the United Auto Workers (UAW) union said of the allegations in a statement. 

"This is despite signing a document indicating they may face termination or criminal prosecution for speaking publicly, or to the media, about what they observed at work or their working conditions."

Tesla must respond to the NLRB charges by September 14. On November 14 the board will hold a hearing before an administrative law judge in Oakland.

"These allegations, which have been filed by the same contingent of union organizers who have been so outspoken with media, are entirely without merit," Tesla said in a statement. "We will obviously be responding as part of the NLRB process."

The UAW is trying to unionize the 10,000 Tesla workers at the Fremont plant, alleging the company has a poor safety record -- a charge it vehemently denies.

"For as long as I’ve been at Tesla, it’s been clear to me that it’s up to the workers to make sure that we’re safe and treated fairly," said Jonathan Galescu, a Tesla production associate, in the UAW statement.

"I joined others in filing the charges for myself, but I also did it for my coworkers -- they need to know we have rights, and that we can speak up about what we are seeing and experiencing," he said.

Tesla founder Elon Musk has criticized the union drive and denied charges that working conditions at the Fremont facility are unsafe.

But he has acknowledged the launch of the company's new Model 3 requires a steady increase in the tempo of production.