Japanese Prime Minister Shinzo Abe said yesterday that he cannot accept Seoul's demand that Tokyo take further measures - including another apology - over its wartime use of " comfort women".
Under a "final and irreversible" deal struck in 2015, Tokyo gave an apology and 1 billion yen (S$12 million) for a foundation set up to support Korean comfort women who are still alive.
Japan, Mr Abe said, "has been carrying out all the promises it made in good faith" and urged South Korea to "firmly follow through" on the pact. "It is a universal principle that bilateral agreements are followed," Mr Abe said yesterday.
South Korean President Moon Jae In, in his New Year press conference on Wednesday, said that the 2015 deal had come about the "wrong way" and urged Japan to make a "heartfelt apology" to the victims, even as he conceded that the pact to end the dispute was not renegotiable.
South Korean Foreign Minister Kang Kyung Wha said on Tuesday that Seoul will not use any more of Tokyo's money for the surviving comfort women, and would fund the reparation from its own budget, prompting Japan to lodge a protest.
Mr Moon said a day later that Seoul will decide how to handle the money offered by Japan in consultations with the Japanese government and other relevant parties.
In his first public response to Mr Moon's comments, Mr Abe said yesterday: "Such unilateral demands that Japan take additional steps are absolutely unacceptable."
Top Japanese government spokesman Yoshihide Suga said Japan "isn't thinking of moving even a millimetre on the deal", while Foreign Minister Taro Kono said any attempt to revise the deal would render bilateral ties "unmanageable".
The highly emotive issue has poured cold water over improving bilateral ties between Tokyo and Seoul, as the two US allies seek closer cooperation on regional security threats such as North Korea.
Between 80,000 and 200,000 women, mostly from the Korean Peninsula, had been recruited to provide sex to the Japanese Imperial Army during World War II.
There were 47 surviving South Korean comfort women at the time the deal was struck, but this figure now stands at 33.
Japan bristles at its neighbours continuing to resurface historical wartime issues even after its repeated efforts to resolve them once and for all. Tokyo says it should not be strong-armed into a form of "apology diplomacy".
In 1965, it struck a deal with South Korea when the two nations normalised ties to completely settle all civilian-related issues, including the comfort women and requisitioned workers during the war.
And in 1993, Tokyo extended "its sincere apologies and remorse to all those, irrespective of place of origin, who suffered immeasurable pain and incurable physical and psychological wounds as comfort women".
Political scientist Tosh Minohara of Kobe University told The Straits Times that the comfort women issue has been dug up over and over again for "domestic political gain" in South Korea.
On President Moon's comments on Wednesday that he seeks "forward-oriented ties" with Japan despite differences over the comfort women issue, Dr Minohara said: "This is like punching someone in the face and then saying to them, 'I hope it doesn't ruin our relationship.' It just doesn't work that way and Japan needs to stand firm."
International relations professor Kim Jae Chun of Sogang University warned that the difference in opinion over the comfort women deal could pose a major roadblock for Korea-Japan relations.
He said South Korea's two-track approach of separating history from politics does not seem to be working, as Japan is likely to feel betrayed by Seoul. "Japan thought that by striking this deal in 2015, it could put the past away and move forward. But the South Korean government has no closure."
•Additional reporting by Chang May Choon in Seoul