A South Korean navy boat fired warning shots Tuesday after a North Korean patrol vessel crossed onto the southern side of the tense Yellow Sea border, according to Seoul.
South Korea's military said the North Korean boat quickly retreated to its side of the so-called Northern Limit Line off the countries' western coast following the warning shots.
There were no indications whether the incursion, which occurred at about 10:20 a.m. local time, was intentional. North Korea's government has not commented on the incident.
Such incursions are not uncommon, since North Korea has never recognized the de facto Yellow Sea border, which was drawn by the United Nations at the end of the Korean War.
The small-scale border incidents can sometimes erupt into more serious clashes, such as in 2010, when North Korea shelled the South's Yeonpyeong Island, killing four people.
The two countries remain in a technical state of war, since the 1950s conflict ended in a truce rather than a peace treaty.