The South Korean Defense Ministry on Tuesday said a North Korean patrol ship crossed the Northern Limit Line (NLL), the western maritime border between the two Koreas, several times on Monday night before retreating after warnings.
Kim Min-seok, the South Korean Defense Ministry spokesman, said the ship crossed the maritime border three times, just west of South Korea's Yeonpyeong Island.
Yeonpyeong Island was the site of North Korean shelling in 2010 which killed two South Korean civilians and two soldiers.
"We believe the North's trespassing of the NLL (Northern Limited Line) is a part of military drills or an inspection of the South Korean military. We are closely monitoring the possibility of the North's provocation by an intentional crossing of the sea border," said Kim.
It comes as South Korea and the United States began their annual joint military drills on Monday despite the North's repeated warnings.
The trespass also occurred during the second round of reunions of families separated by the 1950-53 Korean War, which took place at the Diamond Mountain resort in North Korea.
The two rounds of family reunions took place from Thursday to Tuesday even though North Korea had previously threatened to cancel the reunions should South Korea hold joint military drills with the US. AP