November 29, 2018 4:57 AM EST

A diplomatic spat between South Korea and Japan deepened on Thursday after the Supreme Court in Seoul ruled that Mitsubishi Heavy Industries Ltd. was liable to pay compensation to victims of forced labor in two court cases dating back to World War II.

The court said Thursday that Mitsubishi Heavy must pay between 100 million won ($89,000) and 150 million won ($134,000) to each of five plaintiffs who had sued over being forced to work for a military factory during the 1910-1945 colonial period. The court also upheld another lower court ruling that Mitsubishi Heavy must pay 80 million won ($71,000) to each of five victims of forced labor or their families.

The rulings come about a month after the Supreme Court found Nippon Steel & Sumitomo Metal Corp. liable in a similar case. Japan has condemned the decisions, saying the cases were settled by the 1965 treaty that established diplomatic relations between the two sides — a pact that came with a $300 million payment.

Fraught History

While the compensation amounts in the latest case are easily manageable, the rulings strike at the heart of a fraught history between the two U.S. allies that risks hurting relations in other areas.

“This is a situation that makes it difficult to maintain ties between Japan and South Korea,” Japanese Foreign Minister Taro Kono said. “The South Korean government must take swift steps to remedy the situation,” he added, saying he was very worried nothing had been done since the previous verdict. Mitsubishi Heavy said it would take unspecified “appropriate measures” while communicating with the Japanese government.

According to Japan, 15 similar cases have been filed by South Koreans who say they worked for 69 companies during the Japanese occupation.

Read more: Moon Faces Hard Choice on WWII Case: Anger Korea Voters or Japan

South Korean Prime Minister Lee Nak-yon expressed concern over Japan’s “extreme” reactions to the local court ruling, issuing an unusual statement that said Japanese officials were creating a diplomatic row.

Japan’s reactions continued to be “excessive,” Noh Kyu-duk, spokesman for South Korea’s foreign ministry, said Thursday. He asked Japan to restrain its “regrettable” response. Noh repeated that Seoul was working on measures to respond to the court’s forced labor rulings, but didn’t provide details.

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