Beijing is jeopardising peace in a row with Tokyo over disputed islands, Japan's defence minister said Tuesday, days after China warned any bid to shoot down its drones would constitute "an act of war".

Itsunori Onodera's comments are likely to further heighten fears that the two countries could be sliding towards conflict over the outcrops in the East China Sea and come on the day that China showed off its fleet of nuclear-powered submarines.

"I believe the intrusions by China in the territorial waters around the Senkaku islands fall in the 'grey zone' (between) peacetime and an emergency situation," Onodera told reporters in Tokyo.

The two sides have been at loggerheads over the island chain, which China claims and calls the Diaoyus, since Tokyo bought three of them from their private Japanese owner in September 2012.

But the comments from Onodera following those from China's defence ministry at the weekend, appear to have taken the verbal fisticuffs to a new level.

On Monday, China's coastguard sent four vessels into the waters around the islands, where they stayed for two hours, shadowed by their Japanese counterparts.

That came after three consecutive days in which Tokyo scrambled jets to meet Chinese aircraft as they traversed a strait leading to the Pacific. They did not enter Japanese airspace.

"They were two early-warning aircraft and two bombers," Onodera told reporters on Tuesday.

"It was unusual that so many aircraft flew between the Okinawan main island and Miyako island. We consider that it is also very unusual that it occurred for three days in a row.

"We understand that it is one of the trends showing that China is now vigorously expanding its areas of activities, including into the open ocean."

Tensions likely to increase further

Last week it was reported that popular Prime Minister Shinzo Abe had given the green light to plans to fire on any unmanned aircraft that did not heed warnings to leave Japanese airspace.

The report came after an officially unidentified drone was logged on a trajectory towards southern Japan. Privately, policy-makers said there was no doubt it had been Chinese.

China's defence ministry said on Saturday that Japan's firing on its aircraft "would constitute a serious provocation, an act of war of sorts".

"We would have to take firm countermeasures, and all consequences would be the responsibility of the side that caused the provocation," it said.

Observers warn that the frequent presence of armed vessels and aircraft in the area raises the risk of a confrontation, and point out that a minor slip by a crew member on either side could quickly escalate.

Akira Kato, professor of defence and security at Obirin University in Tokyo said the two sides appeared to be painting themselves into a corner, and without a diplomatic channel of communication, things could only get worse.

"Since China is unlikely to tone down its stance, tensions are likely to increase further," he said.

"For Tokyo, the Japan-US security alliance is the fallback," he said, referring to the security pact under which the United States must come to Japan's aid if it is attacked.

"The case will be a crucial test to see if the alliance can function."

On Tuesday Beijing put its nuclear submarines on display, with state media touting the move as unprecedented and necessary to show other countries the extent of China's strike capabilities

Tokyo announced last week that it was planning a huge drill on an island hundreds of kilometres (miles) away, starting Friday, intended to sharpen the skills of 34,000 troops in defending -- and retaking -- distant territory.

The recent manoeuvres were the latest in a long line of actions and reactions in the bitter scrap with China, which is putatively about the uninhabited islands but is fuelled by historical animosities and nationalism.

Japan says it occupied previously unclaimed islands in 1895. China says it has owned them for centuries and their 19th Century annexation heralded the start of Japan's expansionist imperialism, which culminated in World War II.