A South Korean appeals court on Thursday ruled in favor of a group of former "comfort women" seeking compensation from Japan's government over their treatment in Japanese military brothels during World War II, in a development that could put a damper on improving bilateral relations.

The Seoul High Court overturned a 2021 lower court ruling that dismissed the case on the grounds that South Korea has no jurisdiction over the case due to "sovereign immunity" -- a concept under international law that a state is immune from the jurisdiction of a court in another country.

Plaintiff Lee Yong Soo reacts to a Seoul High Court ruling outside the court in Seoul on Nov. 23, 2023. (Kyodo)

The high court rejected the application of the concept, saying acts by the Japanese military, such as abducting and forcing women to have sexual intercourse with its personnel, violated international treaties to which Japan was a party at the time, as well as Japan's criminal law.

It also pointed out that customary international law, or general practice accepted as law, does not recognize the sovereign immunity of the perpetrator country when it comes to illegal acts suffered by a citizen within their own country.

The court ordered the Japanese government to pay 200 million won ($154,000) in compensation to each of the former comfort women involved in the suit brought by a group of 16 plaintiffs, most of whom are bereaved family members.

Thursday's ruling came as South Korea-Japan relations have been improving after Seoul pledged to resolve a dispute over alleged forced labor at Japanese firms during Japan's colonial rule of the Korean Peninsula.

"I think Japan must pay sincere apologies to the plaintiffs and compensate according to the ruling," Lee Yong Soo, 94, the only surviving former comfort woman among the plaintiffs, told reporters.

Japan lodged a strong protest to South Korea over the ruling.

Foreign Minister Yoko Kamikawa said in a statement that the judgment is "extremely regrettable and absolutely unacceptable," urging South Korea to "immediately take appropriate measures to remedy the status of its breaches of international law on its own responsibility as a country."

Masataka Okano, the Foreign Ministry's top bureaucrat, summoned South Korean Ambassador Yun Duk Min to protest, the ministry said.

Plaintiff Lee Yong Soo (C) and others hold a press conference in Seoul on Nov. 23, 2023. (Kyodo)

The Japanese government declined to be involved in the case and similar lawsuits filed by other groups of South Korean women who claimed to have been forced to work in the military brothels.

Japan has said all issues stemming from its 1910-1945 colonization of the Korean Peninsula were settled "completely and finally" under a 1965 bilateral agreement and that an accord reached by the two countries in 2015 "finally and irreversibly" resolved the comfort women issue.

Japan's argument "could be a point of contention," the high court said Thursday, but added that it was not taken into consideration because the Japanese government did not present its plea.

In April 2021, the Seoul Central District Court dismissed the plaintiffs' case, prompting them to appeal the ruling to the high court.

The lower ruling was contrary to a decision made three months earlier by the same court, but under a different panel of judges, that ordered the Japanese government to pay damages to a group of former comfort women and their bereaved families.

The January 2021 ruling went on to be finalized without Japan appealing it.

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