China risks triggering unintended conflict with Asian rivals through its aggressive stance in maritime disputes, Japan said Tuesday in a security assessment, as a Beijing minister urged preparations for a "people's war at sea".

China's sweeping claims over the South China Sea, where it has built a series of artificial islands capable of supporting military operations despite overlapping claims from other nations, have stoked international alarm.

The region's superpower "continues to act in an assertive manner" and its actions "include dangerous acts that could cause unintended consequences", Tokyo said in a defence white paper.

Beijing is under pressure to respect a UN-backed tribunal's finding last month that there is no legal basis for its ambitions over the South China Sea where the Philippines, Vietnam, Malaysia and others also have claims.

The white paper said China was "poised to fulfil its unilateral demands without compromise" including efforts "to turn these coercive changes to the status quo into a fait accompli".

And it again called on Beijing to abide by the ruling of the tribunal, which China has denounced as a fraud.

Chinese state media in Beijing quoted Defence Minister Chang Wanquan as urging preparations for a "people's war at sea" to counter offshore security threats and safeguard sovereignty.

Chang "called for recognition of the seriousness of the national security situation, especially the threat from the sea", Xinhua news agency said.

The military, police and people should prepare to mobilise to defend national sovereignty and territorial integrity, he was quoted as saying during a tour of the coastal province of Zhejiang.

The agency did not elaborate on the source of the threats.

The United States has said it will continue naval patrols close to reefs and outcrops claimed by China to assert the principle of freedom of navigation, a move which has angered Beijing.

In its white paper Japan also expressed concern over increased activity in the East China Sea, where the two countries have competing claims to small uninhabited islets called the Senkakus in Japan and the Diaoyus in China.

"Recently, China has been intensifying activities near the Senkaku Islands, such as its military aircraft flying southward closer to the islands," it said.

In the year to March 2016, Japan's air force scrambled jets 571 times against Chinese planes flying near Japanese airspace, an increase of 107 from the previous year, it added.

- 'Irresponsible remarks' -

China lodged a "solemn" protest with Japan over the defence white paper, state broadcaster CCTV said in Beijing.

Xinhua, in a report from Tokyo, blasted the paper and accused Japan of "making irresponsible remarks on China's national defence and China's normal and legal maritime activities in the East and South China Seas".

Last month the two countries were at loggerheads over accusations Japanese warplanes locked their fire control radar onto Chinese aircraft.

Beijing sparked alarm after it unilaterally established an air defence identification zone in the East China Sea in 2013, demanding all aircraft submit flight plans when traversing the zone, which covers islands disputed with Tokyo and also claimed by Taipei.

US Defence Secretary Ashton Carter said in February that China's military presence in the South China Sea was increasing the risk of "miscalculation or conflict".

And tensions have also grown over Indonesia's Natuna Islands in the South China Sea, where Chinese and Indonesian boats have clashed.

Japan's white paper also highlighted concerns over North Korea's nuclear programme, saying it was possible it has "achieved the miniaturisation of nuclear weapons and has developed nuclear warheads".

Since carrying out a fourth nuclear test in January, North Korea has claimed it miniaturised a nuclear warhead to fit on a missile and successfully tested an engine designed for an inter-continental ballistic missile that could reach the US mainland.