China's military modernisation, including apparent plans for a deep-water navy, needs to be monitored closely to determine whether it exceeds requirements for self-defence, a Japanese defence report said on Tuesday.
The annual defence white paper, which comes at a time of strained Sino-Japanese ties, echos a US Defence Department report in July that said China's fast-modernising military could pose a long-term threat to the region.
Defence Minister Yoshinori Ohno said, however, that Japan did not see China as a military threat.
"But there are issues that require attention...and we urge China to improve its transparency" on issues such as military spending, Kyodo news agency quoted him as telling reporters.
The report, compiled by the defence ministry, said the Japanese public had become "exceedingly concerned" about the activities of Chinese vessels.
The report noted the intrusion of a Chinese nuclear-powered submarine into waters off Japan's Okinawa islands in November as an example of increasing Chinese naval activity close to Japan.
"Including the incident ... involving the Chinese nuclear-powered submarine, Chinese naval vessels have been navigating in waters near Japan in recent years," it said.
China said the submarine entered Japanese waters by mistake.
Japan has been urging China to explain the reason for the increase in marine activity in hopes that greater transparency will deepen trust, the white paper said.
Japan is also concerned about Chinese gas exploration in the East China Sea near what Japan considers to be its exclusive economic zone, the report said.
Japan must be on guard against such activities, partly since China is said to be striving to expand its navy's reach, it said.
"Regarding the pick-up in China's maritime activity, the trends need to be watched since...it has also been pointed out that the Chinese navy is aiming in the future to become a so-called 'blue-water navy'," the white paper said.