Less than two months after Seoul officially lodged a complaint over another incursion, South Korea scrambled military jets to intercept a Chinese aircraft that entered the South's air defense zone last week.
The Chinese aircraft was believed to be a military reconnaissance plane and spent nearly four hours Saturday in the Korea Air Defense Identification Zone, the South Korean Joint Chiefs of Staff said in a statement.
It would be at least the third such incident this year. South Korea summoned the Chinese ambassador in February to formally complain.
China's Defense Ministry did not respond immediately to a request for comment Saturday, the start of a long holiday weekend.
The Foreign Ministry in Beijing said in February the flights were completely in line with international law and practice and that "air defense identification zones are not territorial airspace."
The Chinese Air Force is increasingly active far from the country's shores, including flying missions into the Western Pacific that often pass through the Miyako Strait in Okinawa Prefecture and around self-ruled Taiwan, claimed by Beijing as its own.
China generally says that it has no hostile intent.
However, the government has said drills near Taiwan are aimed at sending the democratic island the message that there will be consequences if it tries to push for formal independence.