A North Korean patrol boat briefly crossed into South Korean territory on the western side of the peninsula Friday but turned back after South Korean forces fired warning shots.

South Korea’s Joint Chiefs of Staff said that the incursion into the South’s side of the Northern Limit Line in the Yellow Sea took place a little after noon. The South fired six warning shots, and the Northern vessel retreated shortly thereafter.

Naval incidents in the west of the peninsula are not unusual, as the North does not recognize the NLL and has since 1999 claimed a “West Sea Military Demarcation”