A North Korean patrol boat trespassed into South Korean waters on Thursday before returning to the North after nearly an hour.

A South Korean naval vessel guarding the Northern Limit Line (NLL) sent warning messages when the North Korean boat attempted to cross the maritime demarcation line. It then issued another warning when the North Korean boat traveled one-point-six kilometers into South Korean waters, some 12 kilometers off the South’s island of Yeonpyeong.

A military official said the North Korean boat apparently crossed over the border while chasing three Chinese boats that were fishing illegally.

Thursday marked the third time this year for a North Korean boat to violate the maritime border in the Yellow Sea.

The South Korean military said though Thursday’s intrusion seems to have been an accident, the possibility that the move was intended to raise tensions also cannot be ruled out, given that Thursday’s violation was similar in form to past trespasses.