A North Korean artillery battery on Thursday evening fired two shells in the direction of a South Korean missile cutter that was performing routine duties about 10 km south of the sea border, the Joint Chiefs of Staff here said.
The shells fell some 150 m from the boat.
According to the JCS, several North Korean Navy boats were on standby near the Northern Limit Line at the time and fighter jets were scrambled.
The cutter fired five shells in response into waters near a North Korean patrol boat 2 km north of the NLL, and South Korean fighter jets made a sortie in the direction of the NLL.
Twenty minutes after the initial attack, the South Korean Navy escorted some 20 trawlers that were fishing nearby to safety and ordered the residents on nearby Yeonpyeong Island to evacuate. Some 790 of the island's 1,395 residents rushed to 11 air-raid shelters.
On Tuesday, a North Korean ship and several patrol boats strayed into South Korean waters 1.1 km south of the NLL. They turned back after the South Korean Navy fired warning shots.
On Wednesday, North Korea's southwestern frontline command denounced the warning shots as a "military provocation" and warned that South Korean naval ships will be shooting targets.
The South Korean military on Thursday evening sent a telephone message to the North warning against any further provocations.
A military source said, "No matter what intention the North had, it was a dangerous situation which could have damaged the missile cutter."
Military authorities believe the North has no intention of making the situation worse since it fired no further shells.
Military authorities concluded the chance of the North launching any further aggressive provocation is slim.