Chinese ships are seen operating in waters near the Northern Limit Line in the West Sea.
The South Korean military is investigating possible security lapses in connection with a fishing boat's crossing into North Korean waters over the weekend.
According to officials from Seoul's Joint Chiefs of Staff, the 4.5-ton South Korean vessel crossed the Northern Limit Line on Saturday, near the northwestern island of Yeonpyeong, at around 1 p.m.
It stayed in the North's waters for about 10 minutes before returning to the South.
Critics say the maritime crossing is yet another example of laxed maritime defenses, arguing that it took nine minutes for the Navy to identify the vessel after first spotting it heading north around 12:45 p.m.
The military reportedly called the ship some 50 times through a maritime communication line to order its return but it did not answer.
The Coast Guard said a Chinese and two Vietnamese crewmen were aboard the ship, and they apparently crossed the NLL because they were not familiar with the area.
North Korea was immediately informed of the accidental crossing but gave no response.
<Photo: Yonhap News>
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