South Korea's top automaker Hyundai Motor Co. again stood to lose billions of won in sales and output as its unionized workers continued their partial strike for a fourth day Friday.

Some 13,000 workers at the company's main plant in Ulsan walked off the job at 9 a.m., while an additional 15,000 originally scheduled to start their shift later in the day were set to join the earlier shift in a protest visit to the company's headquarters in Seoul.

The company said four-hour strikes over the past three days have already cost some 3,500 cars in reduced output worth 79 billion won ($69.55 million).

The extended walkout comes after the latest and 14th round of negotiations between the company and its unionized workers again fell through Thursday.

Both the management and labor union note the sides remain far apart on most thorny issues that include a pay raise and better working conditions.

The automaker has faced a labor strike every year after the union was formed in 1987, save four. The strikeless years were 1994, 2009, 2010 and 2011.

However, this year's labor strike has introduced a whole new dimension, as the union is also demanding joint negotiations led by an umbrella labor organization, the Korean Metal Workers' Union, the outcome of which would affect workers at not only Hyundai Motor but also its dozen smaller affiliates, including Kia Motors Corp., auto parts maker Hyundai Mobis and Hyundai Steel.

The company refuses to attend such negotiations, noting it is not required to do so by law and that one single wage system cannot be applied to all workplaces.

The company has yet to even offer counteroffers to the union's demand on pay raises and other working conditions.

Apparently recognizing the wide gap between them, the company and unionized workers agreed to resume formal negotiations after summer break, which will end on Aug. 8.

The unionized workers said they will again stage a partial strike on Wednesday. (Yonhap)