A Chinese-owned South Korean automaker on Monday said it would be unable to pay workers on time this month.

About 500 union members protested outside the Ssangyong Motor plant after the firm told staff it was impossible to pay salaries on time due to a lack of operating funds.

A spokesman told Agence France-Presse the monthly payment was due Wednesday but would have to be delayed indefinitely.

Ssangyong said earlier this month that it had asked for a cash injection from Chinese parent Shanghai Automotive Industry Corp, the report noted.

The Chosun Ilbo newspaper had said on Monday the request was turned down because the Korean union had called on Chinese executives to resign and  Ssangyong officials declined to comment on that report.

Ssangyong has said it expects a net loss of more than 100bn won ($US77.3m) this year and its sales fell 63% year-on-year to 3,835 units last month. It stopped production on 17 December until the end of the month to reduce inventory.