South Korea's top court has again upheld lower court rulings that ordered a Japanese firm to pay damages to plaintiffs who sued it over wartime labor.

The Supreme Court on Thursday rejected an appeal by Nippon Steel and ordered the company to pay compensation to the bereaved family of a Korean individual who claimed to have been forced to work at a steel factory in Japan during World War Two.

The lower courts had ruled in the plaintiffs' favor. The compensation order has now been finalized.

Last month, the top court ordered three Japanese firms, including Nippon Steel, to pay compensation to wartime workers in five separate cases.

Japan says any right to wartime claims was settled completely and finally under an agreement when the two countries normalized ties in 1965.

Japan's Chief Cabinet Secretary Hayashi Yoshimasa said Tokyo had lodged a protest with the South Korean side over Thursday's ruling.

Hayashi said the rulings clearly run counter to the 1965 agreement, and are extremely regrettable and totally unacceptable.

He said the South Korean government had stated that a government-affiliated foundation would pay such damages in place of Japanese firms under a program announced last March.

Hayashi said he expects Seoul to use the measure to deal with the series of rulings.

In a related move, plaintiffs of a case against Hitachi Zosen said they had asked a court to seize a deposit made by the Japanese firm during the trial. The deposit was intended to prevent the firm's assets in South Korea from being seized.

The plaintiffs say they hope the funds can be used as compensation.