A Chinese military jet entered South Korea's air defense zone without notice three times on Thursday, prompting Seoul's Air Force to scramble fighter jets in response, defense authorities here said.

The aircraft entered the country's air defense identification zone, called KADIZ, at around 10:21 a.m. from an area near Jeju Island and Ieo Island and exited it half an hour later, according to the Joint Chiefs of Staff (JCS).

It again crossed into the KADIZ at around 11:54 a.m. and flew out of the zone at around 12:51 p.m., the JCS said, adding that the aircraft made an another entry at 2:14 p.m. before leaving the airspace at around 3 p.m.

A JCS official said the aircraft is believed to be a Y-9 type reconnaissance plane.

The Air Force deployed fighter jets to track the aircraft and sent a warning message in a "normal tactical" measure, he said.

Later in the day, the South Korean defense ministry called in a military attache at China's embassy in Seoul to lodge a protest and call for measures to prevent any recurrence.

The foreign ministry also said it called in a Chinese official from the embassy here and expressed regrets while urging the taking of appropriate measures.

A Chinese military plane also entered the area a month earlier, and it is the eighth time that a Chinese airplane has entered the KADIZ so far this year.

A section of the KADIZ overlaps with the air defense zones designated by China and Japan, a source of potential tension among the regional powers.

South Korea's military officials say China's move is aimed at testing responses by Seoul and Tokyo.

An air defense identification zone, or ADIZ, is an area of the skies declared by a state for the early identification and location of foreign planes approaching its territory. It is not defined in any international law or treaty. (Yonhap)