Beijing voiced serious discontent and protested the "unreasonable attitude" of Japan toward President Xi Jinping's Berlin speech about Japan's wartime atrocities, the Foreign Ministry said on Monday.

At a Saturday speech in Berlin, Xi elaborated on China's pledge of peace, and he mentioned the historical fact that "Japan's war of aggression against China caused more than 35 million Chinese military and civilian casualties".

The Japanese military slaughtered more than 300,000 people in Nanjing in 1937, Xi noted.

Japanese Chief Cabinet Secretary Yoshihide Suga said on Sunday that "it is extremely unconstructive" for Xi to make such comments on Japanese history while in a third country, Japan's Kyodo News Agency reported.

Suga also refused to recognize the number of people slaughtered in the Nanjing Massacre, saying that the Japanese government has yet to conclude the number of fatalities because of "the various, deferring opinions about the figure", Suga said.

The wartime atrocities by the invading Japanese army - including the Nanjing massacre - are "undeniable historical facts", Foreign Ministry spokesman Hong Lei said on Monday.

Hong warned that there has been a trend in Japan that aims to whitewash and even deny the country's aggressive history, a trend that has stirred tremendous global concerns.

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