Japan has expressed deep regret over what it called an unacceptable ruling by a South Korean appellate court ordering compensation for Korean victims of wartime sex slavery.

Japan's Chief Cabinet Secretary Hirokazu Matsuno said on Friday that the ruling clearly goes against international law and an agreement between the two countries, and Tokyo will continue to strongly request that Seoul properly rectify the situation.

The Japanese official said, however, that bilateral cooperation has never been more necessary amid the harsh strategic environment following North Korea's repeated projectile launches using ballistic missile technology.

Thursday's ruling by the Seoul High Court ordering Tokyo to compensate 16 of the victims and family members prompted the Japanese foreign ministry to summon South Korean Ambassador Yun Duk-min to lodge a formal protest.

An official at Seoul's foreign ministry, meanwhile, said on Friday that the government respects the 2015 bilateral deal on the sex slavery issue as the two sides' formal agreement.