The shifting relationship between China's one-party government and the military is a "risk management issue" for Japan, while North Korea poses a "significant threat," Tokyo warned Tuesday.
In its annual defence report, Japan said the Chinese People's Liberation Army had been speaking out on foreign policy more frequently, a key shift in political-military ties that has set alarm bells ringing in Tokyo.
"Relations between the (Communist Party) leadership and the People's Liberation Army (PLA) have been getting more complex," said the report published Tuesday, calling the shift a "risk management issue".
"The degree of military influence on foreign policy decisions has been changing," it added.
Senior Chinese military officials have become more vocal, making public comments about US military drills in regional waters, for example, Japanese defence officials said at a briefing.
China has been embroiled in separate spats over regional territorial claims -- with Japan as well as with several Southeast Asian nations including Vietnam and the Philippines -- which have flared up in recent years.
But the report also said the PLA may have limited influence, with the number of its personnel on key political decision-making bodies declining, as China readies for a once-in-a-decade leadership shuffle.
"As part of Japan's risk management, we recognise that the intention and purpose behind China's actions are becoming less predictable, which is a challenge when we address the country," said Toshinori Tanaka, director of the defence ministry's strategic intelligence analysis office.
Defence Minister Satoshi Morimoto added that "there is a certain degree of wariness, not only in Japan but in the whole of East Asia, as to which direction China will be heading".
Tokyo once again in this year's report described China's response to disputes with neighbours as "assertive", a description that raised eyebrows in Beijing when it was published in last year's report.
"China's (military) moves, together with the lack of transparency in its military affairs and security issues, are a matter of concern," the report added, noting that Chinese defence spending had grown 30-fold in the past two decades.
The report comes days after Japan's Prime Minister Yoshihiko Noda and Morimoto suggested Tokyo could use force to defend disputed East China Sea islands, known as Senkaku in Japanese and Diaoyu in Chinese.
Tensions between the Asian powerhouses rose again earlier this month after Chinese vessels twice entered waters near the resource-rich disputed islands, sparking a diplomatic row.
Tokyo's comments about possibly buying the islands from their private Japanese owner generated an angry protest in Beijing.
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.
The report also warned over Pyongyang's high-profile rocket launch earlier this year, which the reclusive state described as a peaceful satellite launch, but critics condemned as a long-range ballistics missile test.
"North Korea is working hard to develop weapons of mass destruction and ballistic missiles," it said, adding that such moves are "raising tensions on the Korean peninsula and are a severe destabilising factor for East Asian security."
The paper said a US regional military presence was a "deterrent" against aggression, and "brings a sense of security to countries in the region".