South Korea’s military said it fired warning shots Friday after a North Korean merchant ship briefly crossed the disputed western sea boundary.

The vessel entered South Korean waters near Baengnyeong Island around 5 a.m. before retreating following audio warnings and gunfire, according to Seoul’s Joint Chiefs of Staff.

No North Korean forces returned fire. Officials said the response followed standard procedures and that South Korea remains on high alert to defend its territory.

The Northern Limit Line has long been a flashpoint, with deadly clashes in 2010 and frequent incursions by North Korean ships.

Pyongyang does not recognize the boundary, instead claiming waters that extend deep into South Korean territory, the Associated Press has reported.

Tensions remain elevated as North Korea advances its weapons program and strengthens ties with Russia while rejecting calls for renewed diplomacy.