Japan could be a key participant if conflict breaks out in Asia, the defence minister said Monday, warning China is seeking to exploit difficulties between allies.

The comments by Itsuno Onodera, who said Japan needs new equipment and must reconfigure its defence, come as Tokyo is embroiled in an ongoing spat with Beijing over disputed territory that has sparked warnings of a possible armed skirmish.

"The crisis that Japan faces now may lead to situations in which the country may have to be involved as a main player," Onodera told a symposium in the capital.

"Before, it was expected that Japan would only be part of a group (involved in any confrontation)," he said, in apparent reference to the US-Japan security alliance.

"Or that a conflict might occur only in areas surrounding the country," he said.

"Japan's defence has been designed for that scenario.

"But Japan (now) needs to have a good defence to protect the country, which can mean equipment, new aircraft, defence systems or cyber protection."

Onodera said Tokyo needed to be wary of China's maritime expansion in the South and East China Sea.

"China has made more and more advancement into the seas," he said.

"When it did not have as much military capability, China tried to promote dialogue and economic cooperation, setting territorial rows aside.

"But when it sees a chance, any daylight between a nation and its ally, it makes blunt advancements. This is what is happening and what we should learn from the situation in Southeast Asia."

Onodera's speech came as he readied to head to Brunei to participate in the ASEAN Defence Ministers' Meeting Plus (ADMM+) starting on Wednesday.

The group gathers defence ministers from Southeast Asian nations and eight other regional powers -- Japan, China, South Korea, the US, Russia, India, Australia and New Zealand.

Onodera said he will "repeatedly explain Japan's position to his Asian counterparts" and that Tokyo's motives were entirely defensive.

Hawkish Prime Minister Shinzo Abe this year boosted Japan's defence budget for the first time in over a decade against the backdrop of growing concerns among many countries in the region about China.

But any move to strengthen military capabilities rouses hostility and suspicion in the region, much of which laboured under the brutal yoke of Japanese occupation until the end of World War II.

Since coming to power in December Abe has repeatedly made noises about altering Japan's pacifist constitution, which bars the country from offensive action.

The defence ministry last month published a paper saying Japan needed amphibious units and surveillance drones to protect its outlying islands.

Japan's moves come against a backdrop of increasing Chinese activity in waters far from its mainland coast.

The two countries have spent the last year involved in a dispute over the sovereignty of the Japanese-administered Senkaku islands in the East China Sea, which Beijing claims as the Diaoyus.

Vessels and planes from both sides have played cat and mouse in their seas, with some observers warning a slip from either nation could provoke a military confrontation, with possibly wide-ranging ramifications.

On Monday, Tokyo scrambled fighter jets after a Chinese government plane approached airspace Japan claims as it own.