"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," Yoshimasa Hayashi told reporters.

Japan said 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 a regular briefing.

"We refrain from giving a definite answer as to the intended purpose of the Chinese aircraft's action. However, recent China's 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."