South Korea's Supreme Court on Thursday ordered major Japanese manufacturer Mitsubishi Heavy Industries Ltd. to pay damages to two groups of South Koreans over wartime forced labor.

The rulings follow a similar compensation order by the top court last month against another Japanese firm, casting further clouds over strained ties between South Korea and Japan.

The Japanese government, which views the issue of compensation over Japan's 1910-1945 colonization of the Korean Peninsula as already settled, reacted almost immediately to Thursday's rulings, with Foreign Minister Taro Kono decrying them as "extremely regrettable."

Tokyo takes the position that the issue has been settled under a bilateral accord attached to a 1965 treaty that established diplomatic ties between the two countries. The accord stipulates that issues relating to property and claims between the two countries and their peoples have been settled "completely and finally."

But the top court determined in Thursday's rulings that the right of forced mobilization victims to seek compensation was not terminated by the accord as it is predicated on Japan's "illegal colonial rule," a line of reasoning also taken in its Oct. 30 ruling against Nippon Steel & Sumitomo Metal Corp.

Mitsubishi Heavy Industries said in a statement after the rulings that the top court decisions were "deeply regrettable," adding that it will "take appropriate measures while maintaining communication with the Japanese government about this issue."

The latest rulings upheld the decisions by lower courts that ordered Mitsubishi Heavy Industries to pay damages to two groups, each consisting of five South Korean plaintiffs, over their conscript labor in Japan during World War II.

The plaintiffs in the first group filed a damages lawsuit in Busan in 2000, claiming they were forced to labor in Hiroshima from 1944 and exposed to radiation from the 1945 atomic bombing of the western Japan city.

Their case was dismissed in a district court and subsequently in an appeals court. But in 2012, the Supreme Court in a landmark decision ruled that the right of individuals to seek compensation was not invalidated by the 1965 accord, sending their case back to a lower court.
In July 2013, the Busan High Court ordered Mitsubishi Heavy Industries to pay the plaintiffs a total of 400 million won (about 365,500 U.S. dollars) in damages, prompting the company to appeal the ruling to the top court.

The plaintiffs are all dead and their case has been taken up by their bereaved families.

The plaintiffs in the other group include members of the so-called Korean Women's Volunteer Labor Corps and sought compensation from Mitsubishi Heavy Industries for their wartime labor in Nagoya in central Japan.

In 2015, a high court in the southwestern city of Gwangju ordered the company to pay a total of 562 million won to the plaintiffs.