The South Korean navy fired warning shots Friday to end a brief incursion by a North Korean patrol boat across the sensitive maritime border, Seoul's defence ministry said.
It was the first such boundary violation in four months, and came ahead of Friday's opening ceremony of the Asian Games in the South Korean port city of Incheon, in which North Korea is participating.
The patrol boat came 0.5 nautical miles (around 900 metres) inside the South side of the disputed Yellow Sea border shortly after midday (0300 GMT), a defence ministry spokesman said.
"One of our naval vessels gave a verbal warning by loudspeaker and then fired six warning rounds," the spokesman told AFP.
The North Korean ship returned across the border minutes later, he added.
The maritime boundary, which was unilaterally drawn by the US-led United Nations forces after the 1950-53 Korean War, was the scene of brief but bloody naval clashes in 1999, 2002 and 2009.
The Korean conflict ended in an armistice instead of a peace treaty and technically the two Koreas are still at war.
Yonhap quoted an unidentified military official as saying the North Korean patrol boat had crossed the border to secure an apparently unmanned barge that had drifted into South Korean waters.
"It towed the barge back to the north," the official was quoted as saying.
Earlier this week, a South Korean fisherman found a suspected North Korean drone in his net near a frontline island close to the Yellow sea border.
The wreckage, without a wing, engine or camera, was recovered when it got caught in his net off Baengnyeong island.
In March and April, South Korea retrieved crashed drones equipped with cameras in three different places, including Baengnyeong island and the northern border city of Paju.