The Yonhap News Agency in South Korea reports seven fishing boats from North Korea crossed the so-called Northern Limit Line, the de facto border, last week. Other fishing vessels were seen in South Korean waters since then.
A senior military official told Yonhap the South Korean navy reserves the right to respond accordingly should the violations continue.
"Our military is closely monitoring to figure out the intention of the North Korean boats' border crossings," the official said on condition of anonymity. "If North Korean boats repeatedly cross the NLL for fishing, the military will promptly and sternly respond, without hesitation."
South Korean forces fired warning shots on North Korean boats that crossed the border Friday. North Korea doesn't recognize the de facto border and authorities from Seoul said they hadn't seen any signs of provocation from Pyongyang.
Seoul accused the North Korean government of jamming its military communications equipment during military operations in coordination with the United States in the region in early March. North Korea said the military exercise was a preparation for war.