Japan on Friday hit back at China and South Korea, calling suggestions that they could jointly mark Tokyo's wartime wrongs "utterly unhelpful", as tensions over history shift alliances in East Asia.
The comments came after Chinese President Xi Jinping and his South Korean opposite number Park Geun-Hye, during summit talks on Thursday, reportedly discussed joining forces for the 70th anniversary next year of Japan's defeat in World War II.
At a regular press conference in Tokyo, chief cabinet secretary Yoshihide Suga said such moves were divisive.
"Any attempt by China and South Korea to coordinate in picking apart past history unnecessarily and making it an international issue is utterly unhelpful for building peace and cooperation in the region," he told reporters.
Both countries were the object of Japan's imperialist aggression in the 20th century and China's state-run Xinhua news agency cited Xi as saying Beijing and Seoul could "jointly hold memorial activities".
"Japan believes that such issues should not be treated as diplomatic issues," Japanese Foreign Minister Fumio Kishida told reporters in Tokyo after suggestions Beijing and Seoul could also co-ordinate research into wartime sex slavery.
Around 200,000 women, mainly from Korea but also from China, Taiwan and Indonesia among others, were forced to work in brothels as "comfort women", serving imperial troops as Japan stomped across Asia before and during World War II.
While mainstream Japanese opinion holds that the wartime government was culpable, a small but vocal tranche of the political right -- including Prime Minister Shinzo Abe -- continues to cast doubt, claiming the brothels were staffed by professional prostitutes.
This equivocation irritates Seoul, which sees it as symptom of Japan's lack of penitence.
The China-South Korean summit came after Beijing began publishing what it said were the "confessions" of 45 convicted Japanese war criminals.
China regularly accuses Japan of failing to face up to its history of aggression in Asia, criticism that has intensified under Abe, who won elections in December 2012 and has advocated a more muscular defence and foreign policy stance.
China and South Korea were outraged in December last year when Abe paid homage at Tokyo's Yasukuni Shrine, which honours Japan's war dead including several high-level officials executed for war crimes after World War II.
Both countries have also expressed disquiet over Japan's decision this week to expand the scope of its military, allowing for the first time troops to fight on behalf of an ally under attack.
South Korea and Japan are the two key US military allies in the region, and some observers have pointed to the possibility that Beijing is keen to exploit a rift between them to help counter US President Barack Obama's strategic "pivot" to Asia.