The artillery shells fell into the maritime buffer zone separating the two Koreas, the South's military said. Seoul has just started its annual military drills to better counter provocations from the North.
North Korea fired artillery shells in waters near South Korea late Tuesday, South Korea's Chief of Staff said in a statement on Wednesday.
South Korea's military said the shells didn't land in South Korean territorial waters but fell into buffer zones separating North and South Korea.
Tensions between the two Koreas escalated last week, after the North fired a barrage of artillery shells and flew warplanes near the border area.
Seoul condemned Pyongyang and imposed its first unilateral sanctions in nearly five years.
South Korea kicks off military training, angers Pyongyang
South Korean troops kicked off their annual Hoguk defense drills, which are meant for enhancing performance capabilities to counter the North's nuclear and missile threats, on Monday.
The drills are the latest in series of exercises by South Korea in recent weeks, including joint activies with the United States and Japan.
The latest training drill, due to end Saturday, comes at a time as North Kore steadily ramps up weapons testing.
South Korean lawmakers have said the North has completed preparations for what would be its first nuclear test since 2017, and might conduct it before November 8 US midterm elections.
What do we know about N. Korea firing artillery shells on Tuesday?
South Korea's Joint Chiefs of Staff (JCS) said North Korean military fired roughly 250 rounds of artillery on Tuesday, calling it a "clear violation" of the 2018 military agreement which created buffer zones along land and sea boundaries to reduce tensions between the two Koreas.
"We strongly urge North Korea to immediately halt its actions," the South Korean military JCS said.
"North Korea's continued provocations are actions that undermine peace and stability of the Korean Peninsula and the international community," it added.
N. Korea says shots meant to warn South Korea
North Korea said later that the short were designed to a send a "grave warning" to South Korea in response to its firing of dozens of artillery shells on Tuesday.
The Korean People's Army (KPA) General Staff said the South's "war drill against the North is going on in a frantic manner."
"In order to send a serious warning once again, it made sure that KPA units on the east and west fronts conducted a threatening, warning fire toward the east and west seas in the night of Oct. 18, as a powerful military countermeasure," it said in a statement released by North Korean state news agency, KCNA.