"(We) took normal tactical measures," such as sending a warning message and dispatching Air Force jets to track and monitor it, the JCS added.

More than 10 aircraft, including F-15K fighter jets, were mobilized, a source said later.

It marked the second flight of a Chinese aircraft, believed to be for military reconnaissance, in the KADIZ in a month.

There were similar incidents in January, February and April as well.

South Korea's Ministry of National Defense summoned the defense attache at the Chinese Embassy in Seoul to deliver a protest message.

Choi Hyong-chan, director general of the ministry's international affairs, emphasized that the government takes the repeated entry of Chinese warplanes into the KADIZ "very seriously," the ministry said.

He urged China to prevent such a case from occurring again, the ministry added, but stopped short of revealing the Chinese embassy official's response.

China has reportedly argued that such sorties were part of ordinary military operations.

China's foreign ministry did not announce an immediate, formal position on Wednesday's action.

"I am not aware of the related issue," ministry spokeswoman Hua Chunying said in response to a question during a press briefing.

"But it's common sense that an air defense identification zone is not (sovereign) airspace."   An air defense identification zone, or ADIZ, is territorial sky declared by a state for the early identification and location of foreign planes approaching its territory. It's not defined in any international law or treaty.

The KADIZ near Ieo Island, a Seoul-controlled submerged rock south of Jeju Island, overlaps with the air defense zones designated by China and Japan, a source of potential tension among the regional powers. (Yonhap)