At about 09:50 local time a North Korean patrol boat crossed the northern dividing line in the Yellow Sea, the official said.
The South Korean military fired a warning shot to make the North Koreans leave the area, but the boat, instead of returning back, opened fire in response. The South Koreans had to fire several shots, and the violator retreated, the South Korean military said.
The Northern Limit Line in the Yellow Sea factually is a maritime border between the two Korean states. The UN troops command unilaterally drew the line at the end of the 1950-53 Korean war, when Americans played the main role there.
North Korean authorities have never recognized the Limit Line and insisted on its transfer southward to the Military Demarcation Line, which divides the Korean Peninsula into two parts.