North Korea has begun live-fire artillery exercises near its disputed western sea boundary with South Korea.
Seoul's Defense Ministry spokesman Kim Min-seok said Tuesday it was informed by Pyongyang of the planned drills.
"Although the notified regions are located north of the NLL (Northern Limit Line), the South Korean military is taking safety measures for residents, including control of civilian vessels. We told North Korea we will sternly respond if its artillery shells fall on the side of the NLL. Our military is paying close attention to whether North Korea will conduct its live fire drill, and on the shell falling area, while keeping a full readiness posture."
Reuters quoted a South Korean military official as saying no North Korean shells appear to have fallen so far in South Korean territory.
The North held similar drills last month. Some of the artillery landed in the South's waters, prompting Seoul to fire back across the boundary.
Such drills are not unusual, but typically further strain relations between the two long-time rivals, which are still technically at war.
Tensions are already higher than usual following warnings that Pyongyang may be preparing to conduct a fourth nuclear test.
During a visit to Seoul last week, U.S. President Barack Obama warned that another test would only further isolate the North.
North Korea has conducted nuclear tests in 2006, 2009, and 2013 in violation of United Nations Security Council resolutions.