President Xi Jinping denounced Japan's "barbaric" wartime aggression for creating enormous suffering in China and South Korea in a speech in Seoul yesterday.
Resurgent nationalism in Japan has stoked regional tensions, and Xi and South Korean counterpart Park Geun-hye were both critical of Tokyo.
They used a lunch meeting to attack Japan for recent attempts to play down atrocities committed during the second world war and for lifting the post-war ban on its armed forces fighting abroad.
Xi was speaking on the second and final day of his visit to Seoul, a trip one analyst said both nations had used to firm up an "alliance" on historical issues.
"Park and Xi agreed that it is worrying that Japan's attitude towards revising history continues even as it seeks to expand its right to self-defence," Ju Chul-ki, Seoul's senior presidential secretary for foreign affairs, was quoted by Yonhap News Agency as saying.
Watch: China's president Xi Jinping calls for stability on the Korean peninsula
Xinhua reported that the two leaders called attention "to the reality" that many countries around the world, and half of the Japanese people, opposed ending the ban on its Self-Defence Forces fighting abroad.
Speaking at Seoul National University, Xi noted the Chinese and Korean people had both suffered at the hands of Japan.
"In the first half of the 20th century, Japanese militarists carried out barbarous wars of aggression against China and Korea, swallowing up Korea and occupying half of China," Xi said.
He used similar rhetoric when he met the speaker of South Korea's National Assembly, Chung Ui-hwa.
"Past experience, if not forgotten, is a guide for the future," Xinhua quoted Xi as saying.
Xi repeated his call for the two nations to jointly commemorate the 70th anniversary of the end of war in the Pacific.
Japan hit back at the two nations.
Chief Cabinet Secretary Yoshihide Suga said: "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."
Xi's trip to South Korea comes at a time of growing concern over security in East Asia. On Thursday, the two nations said they opposed nuclear weapons on the Korean peninsula.
But concerns about Japan appeared to dominate the agenda yesterday. In addition to wartime grievances, Japan also has territorial disputes with Seoul over the Dokdo or Takeshima islands, and with Beijing over the Diaoyu or Senkaku islands.
State-run China Youth Daily ran an advertisement designed by a Korean academic that promoted the name East Sea, as Koreans call the Sea of Japan.
Kim Chul-woo, a senior research fellow at the Korea Institute for Defence Analyses, said Beijing and Seoul were forming an "alliance" on historical issues.
"Strategically, Seoul is still close to the United States. Economically, it tilts towards Beijing. On the historical side, Seoul and Beijing are getting along."
Xi also made conciliatory remarks on China's diplomacy at Seoul National University, reiterating that China would remain a peaceful nation and calling for justice and mutual benefits in international relations.
Several agreements were signed between Chinese and South Korean businesses on the second day of Xi's visit. The Korea Exchange Bank and the Bank of China said they would develop financial products based on the yuan, Yonhap reported.