The United Steelworkers (USW) union has sent a letter from its international president Leo Gerard to South Korean president Moon Jae-in raising concerns about Korean-based tyre maker Kumho's union-busting actions toward its workers in the United States.

It asks the South Korean government to intervene and ensure workers are free to exercise their democratic rights.

The USW filed a petition last month with the National Labor Relations Board (NLRB) to conduct a union election for more than 300 workers at Kumho's plant in Macon, Georgia in the US south. The election is scheduled to take place next week.

Kumho Tire responded to the unionisation effort in Macon by requiring workers to attend meetings in which managers have attempted to dissuade them from joining the union, the union claims. The company also has spent tens of thousands of dollars on union busting consultants who have shared misleading and inaccurate information on an anti union website.

"As we understand that Kumho Tire is currently under effective control of a creditors' committee headed by the Korean Development Bank, it appears that the government of the Republic of Korea may bear direct responsibility for the behaviour of the company's management," Gerard wrote.

"Our union is also concerned about the potential impact the actions of Kumho Tire management in the US may have on the relationship between our countries in the context of current discussions over the possible renegotiation of the KORUS free trade agreement.

"We request that you communicate with Kumho Tire as soon as possible to request that their management at the Macon, Georgia plant immediately desist from all anti union activities."

The USW represents 850,000 workers in North America employed in industries that include metals, rubber, chemicals, paper, oil refining and the service and public sectors.