China dismissed Japanese concerns about its defence spending as "ridiculous" on Thursday after Tokyo commissioned its biggest-ever helicopter carrier.

Both sides are boosting their military budgets as they grow increasingly wary of each other's ambitions in the Asia-Pacific region, facing off over a maritime territorial dispute and how to interpret Japan's motivations and actions during World War II.

Japan is uneasy about what it sees as China's growing assertiveness, including through regular double-digit increases in its defence spending, and on Wednesday commissioned its biggest warship since World War II, the helicopter carrier Izumo.

The 248-metre (810-feet) Japanese-built vessel can carry nine helicopters and is aimed at beefing up Tokyo's maritime defences in the East China Sea.

In 2012, China commissioned its first aircraft carrier, the Liaoning, and has said its plans more.

Tokyo has repeatedly called on Beijing to be more transparent about its military outlays but Chinese foreign ministry spokeswoman Hua Chunying hit back, saying that despite Japan's far smaller number of people it spends a hefty amount on defence.

"Japan's population accounts for only about one-tenth that of China," Hua told a regular briefing on Thursday.

"But its per capita national defence spending is about five times that of China," she added.

"Given this, Japan's criticism of China's national defence is quite ridiculous."

It was unclear on what figures Hua based her comparison.

Earlier this month China unveiled a military budget of 886.9 billion yuan ($142.9 billion) for 2015. With a population of 1.37 billion, that equates to about $104 per person.

Japan's defence spending for fiscal 2015 has been set at 4.98 trillion yen ($42.1 billion), or about $329 per capita, just over three times as much as China.

Kyodo news agency said the Izumo cost around 120 billion yen.

Beijing is suspicious of moves by Tokyo to increase its defence profile under nationalist Prime Minister Shinzo Abe, who has called for his country to throw off the constraints of its "peace" constitution imposed by the United States after World War II, which ended 70 years ago this year.

Asked about the Izumo, Chinese defence ministry spokesman Geng Yansheng told a monthly briefing on Thursday: "Due to historical reasons, any move or actions by Japan in the military and security field is worth the vigilance of its Asian neighbours."

Separately, Geng criticised reported comments by the commander of the US Seventh Fleet suggesting it would back efforts by Southeast Asian nations to form a combined maritime force to patrol areas of the South China Sea.

China claims almost all of the South China Sea, a position that conflicts with Brunei, Malaysia, the Philippines and Vietnam, as well as with Taiwan.

US officials have called for a multilateral agreement to end all actions that risk further inflaming tensions in the region, which includes US allies.

"If ASEAN members were to take the lead in organising something along those lines, trust me, the US 7th Fleet would be ready to support," Bloomberg News quoted Vice Admiral Robert Thomas as saying.

"We urge the US side to stop making irresponsible remarks", Geng said, adding it should "respect the efforts made by the relevant countries in finding a peaceful solution" to the issue.

"We hope that the United States will stop their provocative remarks and actions."