Myung-bak also expressed outrage over what he called the "ruthlessness of the North Korean regime."
Minutes later, North Korea responded with a new threat to attack the South and the U.S., calling their joint war drills "yet another grave military provocation."
Earlier in the day, a U.S. carrier and a South Korean destroyer took up position for military exercises in the tense Yellow Sea.
With tensions rising, China jumped into the fray, calling for an emergency meeting in Beijing among the six nations involved in the stalled North Korean nuclear disarmament talks.
Those talks involve the Koreas, the U.S., Japan, China and Russia.
Tuesday, four South Koreans died when the North rained artillery on the small island of Yeonpyeong.
In its defense, North Korea said civilians were used as a "human shield" around artillery positions and lashed out at what it called a "propaganda campaign" against Pyongyang.
The Associated Press contributed to this story.