North Korea on Thursday fired two artillery shells 150 meters from a South Korean patrol ship operating near the Northern Limit Line, a de facto inter-Korean border, sharply ratcheting up tension on the peninsula.
The South immediately fired back, and there were no casualties, Seoul's Joint Chiefs of Staff said.
"At around 6:00 p.m., the North's artillery shells dropped 14 km southwest of Yeonpyeongdo Island, close to a South Korean patrol ship operating in our waters," the JCS said in a press release. "We directed all fishing boats in the area to return home and the residents to evacuate."
The provocative move from Pyongyang came hours after the South Korean Navy's Second Fleet issued a strong warning against a recent series of violations of the NLL, stressing that it would take an "unsparing" response to any provocations.
"The North must immediately stop any military, provocative threat in the West Sea aimed at nullifying the NLL," the Second Fleet said in a press release.
On Tuesday, three North Korean military boats crossed the NLL and returned home after the South fired warning shots. The following day, the North's southwestern military command berated Seoul for the warning shots, threatening to mount a military strike.
Pyongyang has persistently tried to invalidate the NLL, arguing that it was unilaterally drawn by the US-led UN Command after the end of the 1950-53 Korean War. It argues that it should be drawn further south.