At 14:59 (7:59 Latvian time), about 130 artillery shells were fired simultaneously from two emplacements, the South Korean military said, noting that the deal on creating a buffer zone concluded in 2018 between the two countries it was therefore clearly violated.
None of the shells crossed the Northern Limit Line (NLL), the de facto border between the two countries, the military said, adding it had issued several alerts in connection with the incident.
At a summit in Pyongyang, then South Korean President Moon Jae-in and North Korean leader Kim Jong Un agreed to create a buffer zone on land and sea borders to reduce tensions.
North Korea has repeatedly shelled the buffer zone with artillery in recent months.
North Korea has also launched a number of missiles in recent months, including an ICBM that was the most powerful missile test North Korea has ever conducted.
US intelligence agencies believe North Korea is preparing for its seventh nuclear test.
