A Chinese military plane entered South Korea's air defense identification zone (KADIZ) earlier this week, prompting Seoul to scramble a fighter jet in response, military sources said Wednesday.

The Chinese plane, presumed to be a Y-9 aircraft, entered the zone Monday morning in the area south of South Korea's southern island of Jeju and south of the easternmost islets of Dokdo before moving into the Japanese air defense identification zone, according to the sources.

As the Chinese plane approached the zone, South Korea scrambled a fighter jet and raised the issue with China via a military communication line. In response, China said it was carrying out a regular exercise with the surveillance plane, the sources said.

Air defense identification zones are not territorial airspace and are not bound by international law. But a foreign warplane is supposed to make prior notification before approaching them in line with international customs to prevent accidental clashes.

Chinese warplanes entered KADIZ more than 25 times in 2019 alone, according to officials. (Yonhap)