Japan's defense agency says it is concerned about China's expansion of its military forces.
An annual report issued Tuesday also noted Beijing's increasing use of its maritime military forces, citing an incident last November, when a Chinese submarine violated Japanese territorial waters.
The report says Tokyo will keep a close watch on China's current modernization efforts to determine if they go beyond Beijing's self-defense needs. The head of Japan Defense Agency, Yoshinori Ono, says he wants China to become more "transparent" about its military plans.
Relations between Japan and China are at their lowest in several decades, in part because of a feud over gas drilling in the East China Sea.
Japan is also casting a cautious eye on North Korea's nuclear and ballistic missile programs.