North Korea fired a barrage of artillery shells into waters near borders with the South, South Korea's military said.
The South's Joint Chiefs of Staff said in a statement that Pyongyang simultaneously fired around 130 artillery rounds off its east and west coasts.
Seoul's military said the barrage of artillery was a "clear violation" of a military agreement that was reached between the two Koreas to reduce tensions in 2018.
The inter-Korean summit also established a maritime buffer zone, and some of the shells landed there Monday, the South's military said.
North Korea's military said it conducted the firings after detecting dozens of "projectiles" fired in the South neared the shared border, state news agency KCNA reported, citing a spokesperson of the General Staff of the Korean People's Army.
Shells land on the northern side of disputed border between two Koreas
Seoul added that the barrage of shells landed on the northern side of the Northern Limit Line, the de facto border between the two nations in the Yellow Sea.
The South's military added it issued "several warnings" over the barrage of shells fired, without giving any details.
"Our military is strengthening its readiness posture in preparation for emergencies while tracking and monitoring related developments under close cooperation between South Korea and the United States," it added.
North Korea responds to military drills in the South
North Korea claimed the actions were a response to joint South Korean and US artillery exercises in Cheorwon county, near the demilitarized zone between North and South Korea, scheduled to run from Monday to Wednesday.
The firings also come days after the United States, South Korea and Japan sanctioned three North Korean individuals for their work on Pyangong's ballistic missile-related programs.
The sanctions followed renewed North Korean ballistic missile tests. Pyongyang has ramped up nuclear missile programs in violation of the United Nations Security Council resolutions, and this year resumed testing of its long-range intercontinental ballistic missiles for the first time since 2017.