President Xi Jinping leads ceremony remembering tens of thousands of Chinese killed by Japanese troops in 1937.
Japan’s government may halt funds for UNESCO over a UN decision on including documents of the Nanjing massacre.
Yoshihide Suga, Japan’s chief cabinet secretary, described on Tuesday the decision as biased and “problematic” and criticised UNESCO for going ahead despite the conflicting views of Japan and China.
“The government would like to ask for fairness and transparency in the Memory of the World programme 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.”
Japan’s warning prompted a sharp response from China, who called the threat “shocking and unacceptable.”
Hua Chunying, the Chinese foreign ministry spokesperson, said there was nothing wrong with the documents or the application process.
“Japan can threaten to remove funding to the relevant UN body but it cannot rub away its stains from history. The more it rubs, the blacker it becomes,” she said.
“There are various debates on the issue and Japan’s stance is that it is difficult to put a finger on specific numbers.”
China’s top diplomat was in Japan on Tuesday for high-level political talks as Sino-Japanese ties have been frayed by territorial rows and mutual mistrust over China’s growing military assertiveness, as well as Japan’s bolder security stance.
However, ties have thawed somewhat recently.
The bitter legacy of Japan’s military aggression before and during World War II still haunts ties between Asia’s two biggest economies 70 years after the end of the conflict.
According to China government, Japanese troops killed 300,000 people in the massacre.
A post-war Allied tribunal put the death toll at about half that number.
Japan contributed 3.72bn yen ($31m) to UNESCO in 2014, or about 10.8 percent of its total budget.