The South Korean defense ministry said Tuesday the North had notified them of border drills near the Yeonpyeong and Baengnyeong islands.
"Our military is fully prepared," said spokesman Kim Min-Seok, adding that South Korean fishing vessels had already been asked to leave the area.
"If any shell lands on our side of the border, South Korea will respond strongly," Kim said.
North Korea ran similar drills in late March in which more than 100 shells landed south of the border near the Northern Limit Line (NLL). The action prompted South Korea to respond with hundreds of rounds back in the North's water, with the barrage forcing South Korean islanders to take shelter.
The NLL, a disputed maritime border that has been the de facto sea boundary since the 1950-53 Korean War, is an extension of the land border between the North and the South. Stretching into the sea west of the Korean peninsula, it curves north, isolating five remote South Korean-controlled islands.
Local cable news network YTN said South Korean officials on the islands to seek one of the many shelters there.
South Korea's military presence on the islands has been boosted ever since the 2010 bombardment of Yeonpyeng island by the North, which said it was responding to South Korean military drills.
dr/jm (Reuters, AFP, AP, dpa)