South Korean forces returned fire Wednesday after North Korean artillery shells fell into waters near the tense maritime line that separates the two rivals, a South Korean defense official said.
North Korea fired three shells near the Northern Limit Line in the Yellow Sea, prompting the South to fire three shells back, Defense Ministry spokesman Kim Min-seok said.
South Korean forces have been on high alert in the area since a North Korean artillery attack killed four people in November on South Korea's Yeonpyeong island.
Wednesday's shooting was near that island.
Violence often erupts in the contested slice of sea.
Boats routinely jostle for position during crab-catching season, and three deadly naval clashes since 1999 have taken a few dozen lives.
Kim said one North Korean artillery shell is believed to have fallen south of the maritime line, citing a preliminary analysis of the trajectory of the shell.
The line separating the countries was drawn at the close of the Korean War. It is still a fierce point of dispute.
North Korea argues that the line should run farther south.
Seoul believes accepting such a line would endanger fishing around five
South Korean islands and hamper access to its port at Incheon.
The November attack marked a new level of hostility along the contested line. Two civilians and two marines died, and many houses were gutted in the shelling.
The countries remain technically at war after the 1950-53 Korean War.