Beijing has dismissed concerns raised in a Japanese government report over China's military activity as "groundless" and "irresponsible", adding it had raised formal objections with Tokyo.

China's foreign ministry was responding to Japan's annual defence white paper, released Tuesday, which warned of a "lack of transparency" in Beijing's military strategy and described its response to disputes with neighbours as "assertive".

"China strongly opposes the groundless criticism on China's normal national defence development and military activities, and the irresponsible remarks on China's internal affairs," ministry spokesman Hong Lei said in a statement late Tuesday.

China has already made "solemn representations" with Japan, he added.

Japan's defence report said the shifting relationship between China's one-party government and the military posed a challenge for Tokyo, with Beijing's actions becoming less predictable.

Tensions between the Asian powerhouses rose last month after Chinese vessels twice entered waters near the resource-rich disputed East China Sea islands, known as Senkaku in Japanese and Diaoyu in Chinese, sparking a diplomatic row.

The uninhabited outcrops were the scene of a particularly nasty row in late 2010 when Japan arrested a Chinese trawlerman who had rammed two of its coastguard vessels.