China's growing economic and military clout in the Asia-Pacific region and its assertiveness in territorial disputes -- most recently with the Philippines -- has rattled the United States and its allies, and Monday's incident represents a further heightening of tensions.

Japan, Washington's closest ally in the region, said it scrambled fighter jets after the two-minute incursion from 11:29 am (0229 GMT) on Monday by the Y-9 surveillance aircraft off the Danjo Islands in the East China Sea.

More than 24 hours later China is yet to comment, with US National Security advisor Jake Sullivan due in Bejing later Tuesday for talks with top diplomat Wang Yi that will include the South China Sea.

Analysts said China was possibly probing Japan's air defence network, seeking to obtain intelligence and putting pressure on Tokyo as it expands defence cooperation with the United States and other countries in the region alarmed by Beijing's behaviour.

"The violation of our airspace by Chinese military aircraft is not only a serious violation of our sovereignty but also a threat to our security and is totally unacceptable," Japan's chief government spokesman Yoshimasa Hayashi told reporters.

"We refrain from giving a definite answer as to the intended purpose of the Chinese aircraft's action. However, China's recent military activities near Japan have a tendency to expand and become increasingly active," he told a regular briefing.

- Confrontations -

The uninhabited Danjo Islands are a group of small islets also located in the East China Sea off Japan's southern Nagasaki region, and are not disputed territory.

Japanese and Chinese vessels have been involved in tense incidents in other areas, in particular the remote Senkaku islands in the East China Sea claimed by Beijing, which calls them the Diaoyus.

Tokyo has reported the presence of Chinese coastguard vessels, a naval ship and even a nuclear-powered submarine, in the area, and there have been a series of confrontations between Japanese coastguard vessels and Chinese fishing boats.

Two non-military aircraft from China -- a propeller-powered plane and a small drone -- forayed into airspace near the Senkaku islands in 2012 and 2017, according to Japanese broadcaster NHK.

- 'Counter-strike' -

Japan, staunchly pacifist for decades, has ramped up defence spending with US encouragement, moving to acquire counter-strike capabilities and easing rules on arms exports.

Tokyo is also providing funding and equipment such as patrol vessels to countries across the region, and agreed in July on a deal with the Philippines allowing troop deployments on each other's soil.

Manila and Beijing have been involved in a series of confrontations, most recently in waters near the disputed Sabina Shoal in the South China Sea only 140 kilometres (90 miles) west of the Philippine island of Palawan.

Beijing claims the South China Sea -- through which trillions of dollars of trade passes annually -- almost in its entirety despite an international court ruling that its assertion has no legal basis.

China deploys boats to patrol the busy South China Sea and has built artificial islands that it has militarised to reinforce its claims.

The Philippines defence chief Gilberto Teodoro on Tuesday called China the "biggest disruptor" of peace in Southeast Asia.

Yee Kuang Heng, a professor at the University of Tokyo, said the Y-9 in Monday's incident "was likely probing Japan's air defence network, collecting electronic intel such as Japan's radar signals and coverage".

Naoko Aoki, a political scientist at the RAND think tank, said China may be seeking "to pressure Japan as Japan continues to try to both hedge against and engage with China to balance its security concerns with economic interests".

"The area this happened could be of significance. China claims control over a large area of the continental shelf in the East China Sea, and China may be making a point, challenging Japan's delineation method," she told AFP.

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