South Korea lodged a formal complaint with the Chinese embassy on Friday after a Chinese military aircraft entered South Korea’s Air Defense Zone, the third such incident this year.

Reuters reported the Chinese plane entered the air defense zone and “spent more than four hours flying near a submerged rock in the area controlled by Seoul but claimed by Beijing.”

The rock is called Ieodo and has a South Korean oceanic research station on top of it. It is known as the Suyan Reef in China, while older Western maps refer to it as Socotra Rock. Chinese fishermen ply their trade around Ieodo to the occasional annoyance of the South Korean coast guard. Japan’s air defense zone also technically covers the region, but competing Chinese and South Korean claims are responsible for most of the diplomatic fireworks.

Local mythology holds that Jeju fishermen who drown in the area are doomed to spend the rest of eternity on Ieodo. The importance of the reef to Korean culture should not be underestimated, but it also has concrete strategic significance because it could be used as a base to control undersea oil and mineral deposits in the area.

The waters around Ieodo are also worked by South Korean fishermen from Jeju Island, which has been in the news lately due to an unexpected surge of refugees from Yemen.

When South Korea complained about a similar incident in February, the Chinese ambassador replied that China’s aircraft was on a training mission and did not violate international law by entering South Korea’s air defense zones. Air defense zones are not considered legally enforceable territorial claims, but violating them is generally seen as provocative.

The South Korean Defense Ministry summoned Chinese defense attache Du Nong-yi and “sternly” urged China to avoid more air defense zone penetrations in the future. Seoul stressed that it is taking the incident “extremely seriously.”

The Chinese Foreign Ministry nevertheless claimed to be unaware of South Korea’s complaint on Friday, while the Chinese Defense Ministry refused to comment.

The South Koreans scrambled fighter jets to intercept the Chinese plane as it approached Ieodo. Seoul has not released much information about what kind of plane China was flying, but the South China Morning Post cites speculation that it was on a military intelligence-gathering mission to probe South Korea’s air defenses and the American THAAD missile defense system, in line with China’s drive to develop longer strike range for its fighters and bombers. One South Korean media report described the Chinese plane as a new model of mid-range transport aircraft.