The two Koreas exchanged fire across their disputed western sea border on Monday.
It began in the morning with North Korean live-fire drills near the Northern Limit Line.
South Korea's Joint Chiefs of Staff said North Korea fired some five-hundred artillery shells in the waters north of the Northern Limit Line, and nearly a hundred of those fell in South Korean waters near Baengnyeongdo Island, some more than 3-kilometers south of the border.
South Korean forces immediately retaliated, firing some 3-hundred artillery shells across the NLL with K-9 self-propelled howitzers.
They also dispatched F-15K fighter jets to the area.
Thousands of residents on Baengnyeongdo Island, who were forced to take to shelter during the three hours of artillery exchanges, have since returned to their homes.
The Northern Limit Line is the disputed maritime border off the western coast of the Korean peninsula, where several skirmishes have taken place over the years, including the shelling of Yeonpyeongdo island in 2010.
Earlier Monday morning, the North's Command of the Korean People's Army had faxed South Korean Navy's Second Fleet announcing the live-fire drills, the first time it's informed the South of the exercise ahead of time.
The South Korean government interpreted the announcement itself as a threat with hostile intensions.
Seoul's ministry said the military will be closely working with the United States and will be monitoring for more possible provocations.
Seoul's military has activated crisis management operations and already deployed naval convoys near the maritime border to cope with any possible contingency.
The standing committee of South Korea's National Security Council gathered for an emergency meeting Monday evening, to discuss the North's motives behind its latest provocations.
North Korea announced it was merely conducting a regular drill within its own territory.
The committee talked about the possibility of Pyongyang's further provocations and Seoul's plans for a strong and firm response when needed.
Shin Se-min, Arirang News.