The union at South Korea's top automaker Hyundai Motor started a partial strike Wednesday, sparking fears of vehicle shortages caused by robust sales abroad.
The company said its 44,000-strong union downed tools for two hours on Wednesday, demanding higher wages and an end to night shifts. The union staged its first strike in four years last month.
Unionists will stage a three-hour stoppage on Thursday and four-hour strikes on Friday and next Monday and Tuesday. They will stop work for two hours on Friday next week.
The union at Hyundai's affiliate Kia Motors plans an eight-hour strike on Friday and will refuse to work overtime from next Monday.
The labour action came as Hyundai and Kia, which together form the world's fifth largest automaker by sales, posted robust sales in the first half of this year despite the global economic downturn.
Hyundai's first-half sales rose 11.5 percent from a year earlier to 2.18 million units, or 51 percent of its annual target of 4.29 million.
Kia sold 1.39 million units globally in the first six months, up 12 percent from a year earlier.
Hyundai called for a quick settlement of the disputes, saying the company and union should join forces to ride out an unpredictable business environment.
"We must work together to ride out the economic crisis instead of wasting time on wrangling," the company said in a message to workers.
Hyundai predicted steady growth in the second half thanks to robust overseas sales, especially in the United States and Europe.
But analysts said further strikes would hurt sales at both companies because their inventories are already at record-low levels.
A prolonged labour action "means a blow to their third-quarter results", Suh Sung-Moon of Korea Investment and Securities told Dow Jones Newswires
John Krafcik, chief executive for Hyundai's US operations, said last month the company was facing vehicle shortages in the United States after a blistering 10 percent sales increase in the first half.