Japan may halt funding for UNESCO over the U.N. heritage body's decision to include documents relating to the 1937 Nanjing Massacre, a move that Tokyo has protested against because of what it says are questions over authenticity.
The bitter legacy of Japan's military aggression before and during World War Two still haunts ties between Asia's two biggest economies 70 years after the end of the conflict.
UNESCO included the dossier submitted by Chinese organizations in the latest listing for its "Memory of the World" program, which is intended to preserve important historical materials.
A Japanese government source said the committee that made the decision was made up of experts in document preservation, not in evaluating the veracity of historical content.
Chief Cabinet Secretary Yoshihide Suga said it was "problematic" that UNESCO had made its decision despite the conflicting views of Japan and China.
"The government would like to ask for fairness and transparency in the Memory of the World program so that it would not be used for political purposes," Suga said.
"As for Japan's (financial) contribution (to UNESCO), we plan to look into all possibilities and revisions, including halting payments," he told a news conference.
Japan contributed 3.72 billion yen ($31 million) to UNESCO in 2014, or about 10.8 percent of its total budget.
China says Japanese troops killed 300,000 people in the massacre. A post-war Allied tribunal put the death toll at about half that number.
"The (Japanese) government believes there is no denying that some killings of non-combatants and acts of looting took place," Suga said. "But there are various debates on the issue and Japan's stance is that it is difficult to put a finger on specific numbers."
The dossier, covering the period from Dec. 13, 1937 to early 1938, includes court documents from the Allied tribunal and a separate Chinese military tribunal, as well as photographs said to be taken by the Japanese army and film taken by an American missionary.
China has welcomed UNESCO's decision.
"The Nanjing Massacre is a severe crime committed by Japanese militarism during World War II and is a historical fact recognized by the international community," China's foreign ministry said in a statement on Saturday.
"Facts should not be denied and history not re-written."
Some Japanese conservatives periodically say that accounts of the massacre are a fabrication or exaggerated.
Sino-Japanese ties have also been frayed by territorial rows and mutual mistrust over China's growing military assertiveness, as well as Japan's bolder security stance, although relations have thawed somewhat recently. China's top diplomat was in Japan on Tuesday for high-level political talks.