SEOUL (Reuters) - South Korea summoned a Chinese embassy official on Friday to lodge a complaint over a Chinese military aircraft flying in the South’s air defense territory, the third such incident this year, the defense ministry said.

South Korea scrambled fighter jets to intercept a Chinese plane when it entered the Korea Air Defence Identification Zone (ADIZ), the Joint Chiefs of Staff (JCS) said in a statement.

The aircraft spent more than four hours flying near a submerged rock in the area controlled by Seoul but claimed by Beijing, the JCS said.

Chinese Foreign Ministry spokesman Geng Shuang told reporters in Beijing he was unaware of the issue and referred questions to China’s Defence Ministry, which did not immediately respond to a request for comment.

South Korea’s Defence Ministry said it called in Du Nong Yi, a defense attaché at the Chinese embassy in Seoul, and “sternly” urged China to take steps to prevent the incident from recurring.

“Our government expressed our regret and we’re taking this extremely seriously,” the ministry said in a statement.

Chinese warplanes made a similar flight over the South Korean zone in February and April, the ministry said, while at least two similar instances were observed last year.

The ADIZs are not legally binding agreements under international treaties, but Seoul extended its own territory in 2013 to partially overlap with a zone newly declared by China.

China’s expansion included the submerged rock named Ieodo, which Seoul controls with a research station platform built atop it.

In February, Seoul summoned the Chinese ambassador to lodge a complaint, and Beijing said then the aircraft was in a training in line with international law and practice.