"The labor union of the supplier has been occupying the Hwaseong plant in what we deem an illegal fashion," Kia spokesman Michael Choo said.
Last week, the disruption at the factory south of Seoul resulted in lost production of 3,200 vehicles Thursday and Friday, the company said in a statement. Choo said the workers were still occupying the production line's paint shop.
Kia's Hwaseong plant, which accounts for 42 percent of the automaker's total output, is the company's largest in South Korea.
The automaker said the factory was "partially running" Monday.
Vehicles manufactured at the Hwaseong factory include the Sorento sport utility vehicle.
Kia said the workers were seeking a change in status. "But the fact is they work for Kia's supplier, and are not directly employed by Kia Motors," the company's statement said.
Choo had no further details and didn't know how many workers were involved.
South Korea's Yonhap news agency put the number at 400 and said the workers were demanding higher wages, job security and equal working conditions with Kia workers.
Unionized workers at Kia, an affiliate of the country's largest automaker, Hyundai Motor Co., earlier this month accepted a 5.2 percent wage hike in base salary for this year after staging partial strikes for weeks. Kia is 38.6 percent-owned by Hyundai Motor, the world's sixth-largest automaker.
Labor unrest has been common at the two companies. Hyundai's union has gone on.
