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."
South Korean military officials said the North fired 50 rounds of artillery and that none landed 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.
North Korea disputes the Northern Limit Line boundary, saying it was unilaterally imposed and should be drawn further south.
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.
In Beijing, Chinese Foreign Ministry spokesman Qin Gang told reporters, "we hope the relevant parties can take actions conducive to easing the situation and maintaining peace and stability on the peninsula."
Despite being a long-time ally of North Korea, China has put pressure on the country to resume six-nation talks aimed at denuclearizing the Korean peninsula.
And 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.