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 issues of history shift alliances in East Asia.

The comments came after Chinese President Xi Jinping and his South Korean counterpart, Park Geun-Hye, reportedly discussed at their summit in Seoul on Thursday, the idea of joining hands to commemorate 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 targeted during Japan’s wars of aggression in the 20th century, and China’s state-run Xinhua News Agency quoted Xi as saying Beijing and Seoul could “jointly hold memorial activities.”

“Japan believes that such issues should not be treated as diplomatic issues,” Foreign Minister Fumio Kishida told reporters in Tokyo after suggestions were made that Beijing and Seoul could also coordinate research on Japan’s wartime sex slavery.

Around 200,000 women and girls, mainly from Korea but also from China, Taiwan, Indonesia and other countries, were forced into brothels to become “ianfu” (comfort women) to provide sex for Imperial Japanese troops as they 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 by claiming the brothels were staffed by professional prostitutes, and by challenging the government’s official apology, issued in 1993 by former Prime Minister Yohei Kono, by setting up high-profile panels to “review” it without actually changing anything.

This equivocation irritates Seoul, which sees it as symptom of Japan’s lack of penitence.

But China and South Korea did not mention Japan or history issues in the joint statement or at a joint news conference following the reciprocal summit. Seoul may be concerned that cooperating too much with China against Japan could harm the three-way alliance comprising Seoul, Washington and Tokyo formed to denuclearize North Korea.