North Korean patrol boats have violated the Northern Limit Line (NLL) three times this year.

A South Korean military official said Thursday that North Korean ships had violated the de-facto maritime border in the West Sea twice this month and once in March. The official said the ships returned to their side of the border after the South Korean Coast Guards issued warnings.

The official said the North Korean patrol ships apparently crossed the NLL unintentionally while monitoring hundreds of Chinese fishing vessels in the area.

According to the official, North Korea patrol boats crossed over the NLL eight times last year and eleven times in 2006.