North Korea fired around 130 artillery shells into the sea off its east and west coasts, South Korea's military said, in the latest apparent military drill near their shared border.
The rounds were simultaneously fired just before 6am from two separate sites, one on North Korea's east coast and one on the west coasts, the South's Joint Chiefs of Staff said in a statement.
Some of the shells landed in a buffer zone near the sea border in what Seoul said was a "clear violation" of a 2018 inter-Korean agreement designed to reduce tensions.
It said none of the shells crossed the Northern Limit Line, the de facto maritime border between the two countries.
"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.
The South Korean military sent several warning communications to the North over the firing, the ministry of defence said in a statement.
North Korea did not immediately report on the artillery fire but has now said it fired the shells.
The country has been carrying out an increasing number of military activities, including missile launches and drills by warplanes and artillery units.
North Korea's military conducted its firing after detecting dozens of "projectiles" fired in the South near the shared border, state news agency KCNA reported, citing a spokesperson of the General Staff of the Korean People's Army.
"The enemy should immediately cease military actions that cause escalation of tensions in areas near the front lines where visual surveillance is possible," the unnamed spokesperson said, warning that the North would respond firmly and with over whelming military action to any provocation.
The 2018 Comprehensive Military Agreement (CMA) was the most substantive deal to come from the months of meetings between leader Kim Jong Un and then-South Korean President Moon Jae-in.
With those talks long stalled and collapsed in 2019, however, recent drills and shows of force along the fortified border between North Korea and South Korea have cast doubts on the future of the measures.
Pyongyang has fired artillery into the buffer zone repeatedly in recent months. Officials and analysts in Seoul and Washington say the launches may build up to a seventh nuclear test.
South Korea has accused the North of repeatedly violating the agreement with artillery drills this year. South Korea and the US have also stepped up military drills this year, saying they are necessary to deter the nuclear-armed North.
This year North Korea resumed testing of its long-range intercontinental ballistic missiles for the first time since 2017, and South Korea and the US say it has made preparations to resume nuclear testing as well.
