Tokyo criticised President Xi Jinping yesterday for making remarks during his trip to Germany about Japan's wartime atrocities, adding that the Japanese government had lodged a protest.
At a think tank forum on Friday in Berlin, Xi criticised Japan's wartime actions, saying the Japanese military killed more than 300,000 people in Nanjing in 1937 when it occupied the Chinese city, Jiji Press and Kyodo News reported.
Xi also said more than 35 million Chinese people were killed or injured as Japan waged a war of aggression stemming from its militarism, Kyodo said.
"It is extremely unproductive that a Chinese leader makes such remarks about Japanese history in a third country," said Yoshihide Suga, Japan's chief cabinet secretary.
He said the Japanese Ministry of Foreign Affairs had lodged a protest with the Chinese government on Saturday.
History bears heavy on the Sino-Japanese relationship, most weightily in Nanjing. Japan invaded China in the 1930s and the two countries fought a full-scale war from 1937-45.
China says more than 300,000 people were slaughtered by Japanese troops during a six-week killing spree that began on December 13, 1937, in the capital then known as Nanking.
Although some foreign academics put the number of deaths much lower, no respected mainstream historians dispute that a massacre took place. However, a handful of Japanese officials - most recently the chief of Japan's public broadcaster - have denied it happened.
Japan, which was occupied after its defeat and became a vibrant liberal democracy, has issued apologies for its wartime conduct in Asia.
But frequent statements by conservative politicians and public figures seemingly stepping back from the apologies or calling into question issues of historical fact have increased suspicion on the country's motives.
Beijing and Tokyo are also locked in a row over the sovereignty of the Diaoyu, or Senkaku, islands in the East China Sea.
On Friday, China berated Prime Minister Shinzo Abe for reportedly drawing an analogy between the crisis in Crimea and the Beijing-Tokyo islands dispute.