The South Korean Navy fired five machine gun rounds at a North Korean patrol boat that crossed the Northern Limit Line, the de facto maritime border between the two Koreas.
Seoul's military authorities said Sunday the vessel crossed the Western sea border at around 3:30 p.m. on Saturday by some eight hundred meters.
The crossing was made just northeast of South Korea's Yeonpyeong-do Island, the site of a deadly artillery attack by the North in 2010.
The North Korean vessel was reportedly cracking down on Chinese fishing boats and strayed into South Korean waters.
The boat retreated about eight minutes after the shots were fired.
No skirmishes were reported during the process.
North Korea does not acknowledge the Northern Limit Line, which was drawn in 1953 at the end of the Korean War.
The regime wants the border drawn farther south to divide the waters equally between the two Koreas.
The North's Committee for the Peaceful Reunification of Korea called the firing a military provocation.
The organization's spokesman said Sunday the vessel was conducting a routine operation when the "belligerent South" carried out a provocation against the boat.
He warned the incident could spark military confrontation and refuel tensions on the peninsula.
The incident took place as the second round of reunions for war-separated families is being held, raising concerns it could disrupt the conciliatory mood created by the two Koreas.
Connie Kim, Arirang News.