Seoul, South Korea (CNN) -- The South Korean Navy fired warning shots after three North Korean patrol boats crossed the maritime border at the Yellow Sea, an official with the South Korean Defense Ministry said.
The North Korean boats crossed into the South Korean waters at the boundary, known as Northern Limit Line (NLL), at around 4 p.m. local time (3 a.m. ET) on Tuesday. The ships returned to the northern side after a South Korean naval vessel fired ten warning shots.
The NLL is the de facto maritime border, which stretches from the western coast of the peninsula into the Yellow Sea, marking the boundary between the two neighbors. It was set by the United Nations following the Korean War but the North does not recognize it.
Incidents have been known to occur on and around the border, including one earlier this year when North and South Korean artillery batteries exchanged hundreds of shells.