"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," chief government spokesman Yoshimasa Hayashi said a day after the incident over the East China Sea.

Japan 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 Nagasaki Prefecture.

"We understand that this is the first confirmed and announced airspace incursion by a Chinese military aircraft since we started the anti-airspace incursion measures," 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 said.

"The government will continue to pay close attention to China's military activities and will take all possible measures to ensure vigilant surveillance and airspace violation measures."

Deputy foreign minister Masataka Okano summoned China's acting ambassador late Monday to lodge a "firm protest" and call for measures to prevent a recurrence, the foreign ministry said.

The Chinese diplomat said in response that the matter would be reported to Beijing, according to the ministry. Nearly 24 hours after the incident, there was no official comment from Beijing.

China's growing economic and military clout in the Asia-Pacific region and its assertiveness in territorial disputes -- in particular Taiwan -- has alarmed the United States and its allies.

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.

Japan and South Korea have also moved to bury the historical hatchet. Tokyo is also part of the Quad alliance with the United States, Australia and India, a grouping seen as a bulwark against Beijing.

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".