The government criticized South Korea on Tuesday for planning to erect a monument in China to a Korean who fought Japan's rule of Korea last century and assassinated a Japanese governor.
The move adds to the tensions between Japan and its two neighbors already frayed by differing interpretations of history and long-running territorial disputes.
"This is not good for Japan-South Korea relations," Chief Cabinet Secretary Yoshihide Suga said of the proposed monument to Ahn Jung-geun, a Korean independence activist who shot Hirobumi Ito, the first Japanese resident general of Korea, in 1909 in Harbin, northeastern China.
He said Ahn, regarded as a hero in South Korea and China, "is a criminal."
The plan was revealed when South Korean President Park Geun-hye met Monday with Chinese State Councilor Yang Jiechi in Seoul. Park expressed appreciation for China's cooperation with the plan, according to the South Korean presidential office. Details of the plan are not yet known.
Suga said the government will continue to seek acknowledgement from South Korea regarding Japan's position.
The development is likely to frustrate Prime Minister Shinzo Abe, who has failed to hold any summits with the Chinese and South Korean leaders since coming to power last December.
Japan has been unable to arrange such meetings largely due to Chinese and South Korean opposition to what they perceive as Japan's shift to the right under Abe, including moves to revise the pacifist Constitution.
Ito was Japan's first prime minister, serving from 1885-1888.