A group of U.S. senators has urged South Korea’s Hyundai Motor and other automakers to remain neutral and refrain from interfering in efforts by workers to unionize.

The call was made by 33 Democratic senators in a letter to the heads of 13 automakers including Hyundai, Tesla, Rivian Automotive, BMW and Honda.

The letter expressed concerns that some of the companies have illegally attempted to obstruct organized labor efforts, citing reports of such activity by Volkswagen and Tesla as well as Hyundai, whose supervisors allegedly prohibited pro-union materials in non-work areas outside of normal working hours.

The South Korean manufacturer denied the accusations, saying that recent claims regarding Hyundai Motor Manufacturing Alabama are not accurate, and the company remains in compliance with the rules of the National Labor Relations Board.

The senators said that such acts of retaliation hostile to workers’ rights must not be repeated, calling on the automakers to pledge not to interfere with any efforts to unionize at their plants.