South Korea scrambled F-15K fighter jets to intercept a Chinese reconnaissance plane when it entered the Korea Air Defence Identification Zone (ADIZ) at around 7:10 a.m on friday, the Joint Chiefs of Staff (JCS) said in a statement.
The ADIZ is not the subject of any international treaty and is claimed by both China and South Korea.
South Korea summoned a Chinese embassy official to lodge a complaint over a Chinese military aircraft, the third such incident this year, the defence ministry said.
The Chinese jet, believed to be a Y-9 tactical aircraft spent more than four hours flying near a submerged rock in the area controlled by Seoul but claimed by Beijing, the JCS said in a statement.
Choi Hyong-chan, director general of the ministry's international affairs urged Beijing to prevent such a serious case from occurring again, ministry officials said.
Chinese military planes entered the KADIZ in February and April as well, taking similar flight paths, while at least two similar instances were observed last year, the minister said in a statement.
Chinese Foreign Ministry spokesman Geng Shuang told reporters in Beijing that he was unaware of the issue. He said, “Our government expressed our regret and we’re taking this extremely seriously.”
Seoul extended its own territory in 2013 to partially overlap with a zone newly declared by China which included the submerged rock named Ieodo, controlled by Seoul with a research station platform built atop it.