Japan slammed what it called on Tuesday the first confirmed incursion by a Chinese military aircraft into its airspace as a "serious violation" of its sovereignty, saying Beijing was becoming "increasingly active".

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. 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 on Monday morning by the Y-9 surveillance aircraft off the Danjo Islands in the East China Sea.

Chinese foreign ministry spokesman Lin Jian said Beijing was "gathering information and verifying the situation" and that China "has no intention of intruding into the territorial airspace of any country".

However, Japan's chief government spokesman Yoshimasa Hayashi said: "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."

"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 reporters at a regular briefing.

Confrontations

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 alarmed by Beijing's behaviour.

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.

Japan's Yomiuri Shimbun daily cited an unidentified source linked to the Chinese Communist Party as saying that the incursion was in part a "retaliation" for a Japanese destroyer entering Chinese territorial waters off Zhejiang province in July.

The uninhabited Danjo Islands are a group of small islets 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 a deal with the Philippines in July allowing troop deployments on each other's soil.

Manila and Beijing have been involved in a series of confrontations, most recently 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.

US National Security Advisor Jake Sullivan arrived in Beijing on Tuesday for talks with top diplomat Wang Yi that will include the waterway.

Japan said late on Tuesday it "opposes any unilateral attempts to change the status quo by force as well as any actions that increase tensions in the South China Sea, including obstruction of freedom of navigation and over flight".

"Japan has consistently advocated upholding the rule of law at sea and will continue to cooperate with the international community... to protect the free and open international order based on the rule of law," a foreign ministry statement said.