Earlier in the day, a South Korean court ordered Mitsubishi Heavy Industries Ltd to compensate 10 Koreans who worked at its factories in 1944 with no pay and a bereaved family member of another on two separate suits.

The court upheld two appellate court judgments -- one that ordered Mitsubishi to disburse 100-120 million won ($89,000-109,000) to each of four female victims, and the relative, and the other that ordered it to pay 80 million won each to six elderly men.

Kono said the rulings would incur "unjust disadvantages" to the Japanese company and uproot the legal foundation of friendly, cooperative relations between the two nations. He described them as a violation of international law and urged Seoul to take appropriate measures to address the problem.

Otherwise, he warned, Japan will consider "every option," including an international lawsuit.

Chief Cabinet Secretary Yoshihide Suga also said in a press briefing that Japan has "various specific" countermeasures in hand.

He would not elaborate.

Japan's Vice Foreign Minister, Takeo Akiba, summoned Seoul's Ambassador to Tokyo, Lee Su-hoon, to lodge a formal protest.

The court decision was widely expected, echoing its verdict against Japan's Nippon Steel & Sumitomo Metal Corp. a month earlier for its similar wartime atrocity against Koreans.

It made clear that victims have their own rights to compensation despite the state-to-state agreement and the statute of limitations has not expired in relevant cases.

Thursday's rulings usher in the worsening of already frosty bilateral relations, despite Seoul's policy of separating regional security cooperation with Tokyo and historical issues.

"(We) respect the judiciary's decision and will try to get the wounds of forced labor victims healed," a South Korean foreign ministry official said.

He said the government plans to craft ways to handle the issue in comprehensive consideration of various factors.

He added South Korea still hopes for the "future-oriented development" of relations with Japan. (Yonhap)