No shells landed this time, but earlier this week, four South Koreans died when the North rained artillery on the small island.
South Korea is now trying to pinpoint the exact location of Sunday's artillery fire.
Meantime, a top Chinese official made an urgent trip to Seoul for talks with South Korea's president.
The Obama administration has urged China to help defuse the escalating situation.
In its defense, North Korea said civilians were used Tuesday as a "human shield" around artillery positions and lashed out at what it called a "propaganda campaign" against Pyongyang.
North Korea is calling the exercises provocation and warned of retaliatory attacks.
The North's attack in an area with a civilian population marked a new level of hostility along the rivals' disputed sea border. Eight months ago, according to the findings of a South Korean-led international investigation, a North Korean torpedo sank a South Korean warship in waters farther west, killing 46 sailors.
The aggression could be linked to the North's attempt to strengthen its government as it pursues a transfer of power from leader Kim Jong Il to a young, unproven son.
It also may reflect Pyongyang's frustration that it has been unable to force a resumption of stalled international talks on receiving aid in return for nuclear disarmament.
The Associated Press contributed to this story.