The United Auto Workers union said Thursday it had lodged a formal complaint against electric auto maker Tesla, accusing it of firing and harassing employees involved in union activity.

The move follows last month's decision by the National Labor Relations Board (NLRB) to begin proceedings against Tesla for allegedly infringing on worker rights by requiring over-broad confidentiality from employees that could prevent complaints about working conditions and safety.

"Tesla, which recently fired hundreds of employees for 'performance issues,' swept up many pro-union employees in the terminations," UAW said in a statement.

"Several pro-union employees have come forward with copies of their performance reports, which show no areas of concern."

In a complaint filed with the NLRB, the union accused Tesla of firing workers to discourage, or in retaliation for, union activity and of disciplining employees who wore the UAW logo.

The company, which is based in Fremont, California, denied the allegations.

"At Tesla, we strive to be a fair and just company, the only kind worth being. Performance reviews result in promotions and occasionally in employee departures," a spokesperson said.

"No one at Tesla has ever or will ever have any action taken against them based on their feelings on unionization. It's worth remembering that each year."