Japan's chief government spokesman said on Friday that a South Korean court ruling ordering damages to be paid to 12 so-called "comfort women" was regrettable and unacceptable.
The issue related to women recruited into wartime brothels had been settled by a 1965 treaty between the countries, Chief Cabinet Secretary Katsunobu Kato told reporters. The South Korean government should make an "appropriate response" to the court ruling, he said.
Tokyo's foreign ministry summoned South Korean Ambassador to Japan Nam Gwan-pyo to lodge a complaint.
The ruling could rekindle diplomatic and history feuds between the two countries.
Reminders of Japan's 1910-45 colonial rule are contentious for both sides, with many survivors of "comfort women" - a Japanese euphemism for the sex abuse victims - demanding Tokyo's formal apology and compensation.
Japan says the issue was settled under a 1965 treaty that normalised diplomatic ties, and the two countries agreed to "irreversibly" end the dispute in a 2015 deal.
But the Seoul Central District Court ordered Japan pay each of the women 100 million won ($91,000), saying neither of the pacts can cover their right to seek compensation.
South Korea and Japan, both staunch U.S. allies, are key trade partners and share other common interests, including fending off North Korea's growing nuclear and missile threats.
But relations plunged in recent years as history rows flared, especially after South Korea's Supreme Court ordered two Japanese firms to compensate some wartime forced laborers.
South Korea's President Moon Jae-in effectively nullified the 2015 settlement, in which Japan issued an official apology and provided 1 billion yen ($9.6 million) to a fund to help the comfort women victims.© Thomson Reuters 2021.