South Korea’s Supreme Court on Thursday ordered major Japanese manufacturer Mitsubishi Heavy Industries Ltd. to pay damages to two groups of South Koreans for wartime labor, following a similar order last month against another Japanese company.

The two rulings by the Supreme Court, which each drew an immediate protest from Tokyo, have set a clear precedent for similar cases involving Japanese companies, while affecting ties between South Korea and Japan that are often strained by issues of wartime history.

The rulings “completely overthrow the legal foundation of the friendly and cooperative relationship that Japan and the Republic of Korea have developed since the normalization of diplomatic relations in 1965,” Foreign Minister Taro Kono said in a statement, calling the court decisions “extremely regrettable and totally unacceptable.”

Kono said Japan is urging South Korea to take appropriate steps as the rulings go against international law, adding that failure by Seoul to take action will lead to countermeasures by Tokyo, including taking the cases to an international court.

Chief Cabinet Secretary Yoshihide Suga said at a news conference that the ruling could have a negative impact on Mitsubishi Heavy Industries’ operations, though he did not elaborate further.

The firm said in a statement 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.”

Vice Foreign Minister Takeo Akiba summoned South Korean Ambassador Lee Su-hoon to the Foreign Ministry to lodge a protest.

Akiba told Lee that Tokyo is ready to deal with the issue “resolutely,” according to the Foreign Ministry. Lee told reporters after the meeting that he and Akiba explained the stance of their respective governments.

South Korea’s Foreign Ministry said in a statement that it respects the rulings, adding that it is “very concerned” about “excessive responses” from the Japanese government to judicial decisions in South Korea.

Tokyo takes the position that the issue over wartime laborers, some of whom were recruited or forced to work, 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 rights of victims of forced mobilization 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.

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 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 work in Hiroshima from 1944 and exposed to radiation from the 1945 U.S. atomic bombing of the 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) in damages, prompting the company to appeal the ruling to the top court.

All the plaintiffs in that case are deceased and their cases have been taken up by their bereaved families.

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

In 2015 a high court in Gwangju in southwest South Korea ordered the company to pay a total of 562 million won to the plaintiffs.

At a news conference after Thursday’s ruling, 89-year-old Kim Sung-joo, one of the plaintiffs made to work at the Nagoya factory during World War II, expressed appreciation for all those involved in her case.

“We couldn’t do anything on our own other than suffer, but with all of your efforts, we’ve finally made it today. Thank you,” Kim said in a quivering voice.

Lawyers for the plaintiffs say it is now the South Korean government’s responsibility to take the lead in figuring out how to carry out the court orders in consultation with the Japanese government.

Also Thursday, the Seoul Central District Court upheld a lower court order against Nippon Steel & Sumitomo Metal to pay damages to three relatives of South Koreans mobilized to work in Japan during World War II.

The pair of rulings came about a month after another top court ruling in South Korea that also ordered Nippon Steel & Sumitomo Metal to compensate four South Koreans subjected to work for the company during Japanese colonial rule between 1910 and 1945.

Prime Minister Shinzo Abe called the October ruling “unbelievable” and indicated that Tokyo could take the matter to the International Court of Justice.

Despite the 1965 normalization of diplomatic relations, issues related to wartime history continue to cast a shadow over ties between the two countries.

In another twist in the relationship, Seoul decided earlier this month to disband a Tokyo-funded foundation created under a 2015 bilateral accord to settle the issue of South Korean “comfort women” who provided sex to Japanese soldiers in wartime brothels, including those who did so against their will.

Japan reacted strongly to the move as the December 2015 bilateral deal had been signed to “finally and irreversibly” settle the issue.

Editor’s note: In the past, The Japan Times has used terms that could have been potentially misleading. The term “forced labor” has been used to refer to laborers who were recruited before and during World War II to work for Japanese companies. However, because the conditions they worked under or how these workers were recruited varied, we will henceforth refer to them as “wartime laborers.” Similarly, “comfort women” have been referred to as “women who were forced to provide sex for Japanese troops before and during World War II.” Because the experiences of comfort women in different areas throughout the course of the war varied widely, from today, we will refer to “comfort women” as “women who worked in wartime brothels, including those who did so against their will, to provide sex to Japanese soldiers.”