SEOUL (AFP) - A South Korean court has issued an unprecedented order for assets seized from Japan's Mitsubishi Heavy Industries to be sold to compensate World War II forced labourers, prompting Tokyo to protest on Tuesday (Sept 28).

Japan and South Korea are both democracies, market economies and United States allies, but their relationship has been strained for decades over Tokyo's brutal 1910 to 1945 colonial rule of the Korean peninsula.

Around 780,000 Koreans were conscripted into forced labour by Japan during the 35-year occupation, according to data from Seoul, not including women forced into sexual slavery by Japanese troops.

On Monday, the Daejeon District Court ruled that two patents and two trademarks held by Mitsubishi Heavy should be sold to compensate two female Korean plaintiffs in their 90s, according to South Korea's Yonhap News Agency.

The proceeds are expected to raise enough to pay each victim around 209 million won (S$239,000) in compensation and interest, it said.

It is the first time a South Korean court has ordered the liquidation of Japanese corporate assets in a damages suit filed by World War II forced labourers, Yonhap added.

Tokyo government spokesman Katsunobu Kato described the South Korean court's ruling as a "clear violation of international law".

"The Japanese side strongly requested the South Korean side last night in Seoul and this morning in Tokyo to take appropriate measures immediately," he added.

The 1965 treaty that saw Seoul and Tokyo restore diplomatic ties included a reparation package of about US$800 million (S$1 billion) in grants and cheap loans, and stated that claims between the countries and their citizens were "settled completely and finally".

Tokyo insists that extinguished the victims' right to sue.

The relevant company will immediately appeal the Korean verdict, Mr Kato said.