The presidents of China and the Republic of Korea shared their nations' concerns over Japan's new military policy on Friday, days after Tokyo announced a landmark shift in its military stance.
They also shared their countries' common painful past under Japanese militarism.
Observers said the two leaders' position, with President Xi Jinping's suggestion of joint memorial activities to mark their nations' victory over Japan, signals a subtle change in East Asian geopolitics.
Xi, making a two-day visit to Seoul, met his South Korean counterpart Park Geun-hye at a special luncheon meeting, their second meeting in two days.
His visit follows Japanese Prime Minister Shinzo Abe's announcement on Tuesday that Japan's military has the right to collective self-defence and troop deployment overseas, marking a radical change in the country's postwar pacifist stance.
Ju Chul-ki, senior foreign affairs and security adviser to Park, said the two presidents called attention to the fact that many countries and more than half of the Japanese people oppose the reinterpretation.
Xi and Park also voiced regrets over Japan's recent review of the 1993 apology for wartime sex slavery, saying the review was aimed at "damaging and disparaging" the apology.
The Kono Statement, an official apology made by then Japanese chief cabinet secretary Yohei Kono in 1993, acknowledged that Japan was involved in forcing more than 200,000 women, mostly Koreans, to serve in military brothels.
On June 20, the Japanese Cabinet announced the results of its reexamination of the statement. It said the ROK government intervened in the wording of the apology, indicating this was the result of closed-door political dealings.
Xi and Park also agreed that international coordination to resolve the Democratic People's Republic of Korea nuclear issue can be breached if sanctions imposed on Pyongyang for its nuclear programme are mishandled.
The Japanese Cabinet last week agreed to ease sanctions on the DPRK after Pyongyang formed a team to reinvestigate the whereabouts of Japanese citizens it kidnapped in the 1970s and 1980s.
Xi said in an address at Seoul National University earlier in the day, "When the war against Japan was at its most intense, the Chinese and Korean people shared their suffering and helped each other with sweat and blood.
"Past experience, if not forgotten, is a guide for the future," he said while meeting ROK National Assembly Speaker Chung Ui-hwa. "In line with that spirit, the two countries can jointly hold memorial activities next year."
Next year marks the 70th anniversary of victory in China's War of Resistance Against Japanese Aggression, and of the liberation of the Korean Peninsula from Japan's colonial rule.
Chung said the ROK's National Assembly proposed setting up a joint committee to examine and study the history of the ROK, China and Japan and to ensure that what happened in the past is correctly understood, recorded and recognised.
Seoul's Yonhap News Agency said Xi's remarks "stress the necessity for the two countries to jointly respond to Japan's denial on its crimes committed in war."
Tokyo was quick to respond to events in Seoul.
"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," Yoshihide Suga, chief cabinet secretary, told reporters.
But Huang Youfu, a professor of Korean studies at Minzu University of China in Beijing, said China and the ROK are not merely targeting Japan's war crimes, but its refusal to acknowledge that they occurred.
"Indeed, the ROK and Japan are allies of the US But the fact that the ROK has not agreed to join the National Missile Defence project reflects that it wants to keep a balanced position between the US and China, as its economic interests lie more with the latter," Huang said.
Jin Canrong, deputy dean at the School of International Studies at Renmin University of China, said relations between Beijing and Seoul are "enjoying their best momentum in history."
"This has been nurtured especially in the past year by the similar situation they face when it comes to dealing with Japan," Jin said.
He said the stronger bonds between the two increasingly powerful countries will benefit regional stability.
Jin added that China and the ROK have been careful not to alienate the US or to suggest that their strengthening ties in any way weaken ties with Washington.
Xi also said in his speech that powers outside the region are welcome to share its common prosperity.