A North Korean patrol boat briefly violated the tensely guarded western maritime border on Friday, but it retreated after South Korea fired warning shots, Seoul's Joint Chiefs of Staff (JCS) said, according to Yonhap News Agency.
The boat crossed the Northern Limit Line in the Yellow Sea at around 12:10 p.m. (0310 GMT), but it returned to its territory six minutes later as the South Korean Navy fired six warning shots, the JCS said.
"The military is keeping close tabs on any North Korean military movement, maintaining our war readiness," a JCS official was quoted as saying.
The armed ship followed an unmanned stray North Korean wooden boat across the sea border before returning to the northern side of the border with the stray boat, the report said. It was the first incursion by a North Korean boat following the last sea border violation in late May.
The incident also comes as a North Korean delegation is in the South Korean port city of Incheon to participate in the Incheon Asian Games, which open later Friday.