Seoul and Washington have extended until Saturday their joint air exercises, the largest ever, which involve hundreds of warplanes on both sides.
Pyongyang said the decision to extend aerial exercises was "a very dangerous and wrong choice" and fired three short-range ballistic missiles late Thursday.
Shortly after the announcement, as of 11:28 p.m. local time on Thursday, the Seoul military detected around 80 artillery fire from the North in a maritime "buffer zone" from the Kumkang area of Kangwon province on the coast. east of the country.
The barrage is "a clear violation" of the 2018 inter-Korean agreement which established buffer zones to reduce tensions between the two sides, the South Korean general staff said.
The United States denounced "the illegal and destabilizing firing of an intercontinental ballistic missile", while Seoul and Washington promised to take new measures to demonstrate their "determination and their capacities" in the face of growing threats from the North.
Pyongyang fired around 30 missiles on Wednesday and Thursday, including one that ended near southern territorial waters for the first time since the end of the Korean War in 1953.
South Korean President Yoon Suk-yeol said it was a "de facto territorial invasion.
This show of force by Pyongyang comes at a time when South Korea and the United States are carrying out the largest air exercises in their history in the region.
The exercise dubbed "Vigilant Storm" constitutes "an aggressive and provocative military maneuver targeting the Democratic People's Republic of Korea", denounced the North Korean regime on Wednesday, which threatened Seoul and Washington to "pay the most horrible price of the 'story".
Officials and analysts have been warning for months that North Korea is about to carry out a nuclear test, which would be its seventh.
In addition to the "Vigilant Storm" exercise scheduled until Saturday, the South Korean military announced that it will carry out its annual "Taegeuk" exercise next week aimed at "improving wartime performance" and management. of crisis.
It is a computer-simulated exercise conducted to build "the ability to conduct practical missions in anticipation of various threats such as nuclear weapons, missiles and recent provocations by North Korea," according to the military. South Korean.