North Korea on Thursday fired shells into disputed waters near a South Korean warship on patrol south of the Yellow Sea border, South Korea's defence ministry said.
"The shells fell near our ship which has been on regular patrol in our territory, but it did not cause any damage to our ship," a ministry spokesman told AFP.
The North's move began at 6:00 pm (0900 GMT), prompting a response from the South Korean vessel which fired an unspecified number of rounds into North Korean territorial waters, he said.
The South Korean ship was sailing near the front-line island of Yeonpyeong, the spokesman said, adding residents on the island were advised to move to civilian shelters.
In November 2010, North Korea shelled Yeonpyeong island, killing four South Koreans and briefly triggering concerns of a full-scale conflict.
The North's military threatened Wednesday to attack South Korean warships "without any warning" if there was even a "trifle" violation of the Yellow Sea maritime border.
The threat came a day after a South Korean naval ship fired warning shots to stop an incursion by three North Korean patrol boats across the sea boundary.