(Photo: AFP)

South Korean President Moon Jae-in on Thursday said Japan cannot declare the issue of its former wartime sex slaves "over", repeating calls for Tokyo to apologise over the issue and confront wrongdoings.

The controversy of the so-called comfort women -- those forced into sexual slavery for Japanese troops during World War II -- has marred relations between the neighbours for decades.

Park Geun-Hye, Moon's ousted predecessor, struck a deal in 2015 with Tokyo under which Seoul promised not to raise the issue again and Japan paid 1 billion yen ($8.9 million) to a foundation dedicated to supporting the victims.

The agreement, in which Tokyo fell short of taking legal responsibility, angered some victims and after taking office Moon condemned the deal as a "wrongful" solution and urged Japan to make a "heartfelt apology".

"The issue of the comfort women cannot be declared 'over' by the Japanese government," Moon said in a speech marking the anniversary of the country's 1919 uprising against Japanese colonial rule.

"Wartime crimes against humanity cannot be covered up with a declaration that it is over.

"The true way of resolving a tragic history is to remember that history and to learn from it," Moon said, expressing hopes for strong future relations "with the closest neighbour on the backdrop of a sincere apology".

Moon's remarks drew a swift response from Japan, which termed them "extremely regrettable".

Asked to comment on Moon's speech, top Japanese government spokesman Yoshihide Suga told a regular press conference: "President Moon mentioned the comfort women issue although we confirmed the final, irreversible conclusion in the 2015 agreement between Japan and South Korea."

"It goes against the Japan-South Korea agreement. We cannot accept it at all and feel it is extremely regrettable. We immediately conveyed our stance and made a strong protest to the South Korean side through diplomatic channels."

Mainstream historians say up to 200,000 women, mainly from Korea but also from China, Indonesia and other Asian nations, were forced into sexual slavery for Japanese troops during the World War II.

Many South Koreans remain bitter about abuses under Japan's ruthless 1910-45 colonial rule, including the issue of the wartime sex slaves.

Most Japanese feel they have atoned enough for the country's wartime aggression, including the comfort women issue, after numerous apologies and statements on the war.