A South Korean defense official said Wednesday the North fired three shells near the Northern Limit Line in the Yellow Sea and his country fired back three shells in response. No one was hurt.
South Korean forces have been on high alert in the area since a North Korean artillery attack killed four people late last year.
The firing follows a recent easing of animosity between the Koreas and could be a warning about joint U.S.-South Korean military drills set for next week.
North Korea has not commented.
The Associated Press contributed to this report.