South Korean President Lee Myung-bak Monday apologized for not protecting South Korean civilians from North Korea's artillery attack on an inhabited island, which he labeled an inhumane crime.
The president says North Korea will be made to the pay the price for any further provocation.
Mr. Lee called for national unity in a country split between hardliners demanding immediate punishment and liberals worried that an escalation of tensions could plunge the peninsula into a war.
Such a conflict could result in North Korea firing artillery onto the crowded capital, Seoul, imperiling the South's vibrant economy.
The president made no reference to China's offer to host emergency multi-national talks on the crisis with North Korea. But the South Korean president says it is difficult to expect that Pyongyang will abandon nuclear weapons and military brinkmanship.
Last Tuesday's artillery attack was the second provocative act of the year blamed on North Korea.
South Korea responded with restraint after one of its navy warships was sunk in March in the Yellow Sea, killing 46 sailors. An international investigation concluded the Cheonan was hit by a North Korean torpedo.
Pyongyang denies any responsibility for the sinking. But it does acknowledge bombarding Yeonpyeong island on Tuesday. North Korea says it was protecting its sovereignty after South Korean troops on an annual training exercise fired artillery into a disputed Yellow Sea border area.
The attack on Yeonpyeong killed two South Korean marines and two civilians.
Northeast Asia analyst Daniel Pinkston, from the International Crisis Group, says the two incidents present a dilemma for the South Korean leadership.
"This year there have been two tremendous failures in deterrence. So deterrence must be restored," he said. "However, by demonstrating the capabilities and the cost that North Korea would pay in case of further provocation you want to do that in such a way that North Korea does not misperceive or miscalculate, and in fact you end up triggering a war because a war would be very costly."
In a show of deterrence the U.S. Navy has sent a U.S. aircraft carrier and other ships into the Yellow Sea to train with South Korea's navy.
South Korean vessels and aircraft are taking part in what military officials say is a "live fire" exercise meant to demonstrate the alliance's resolve and capability. The exercise ends Wednesday.
The North is impoverished and isolated, with neighbor China its only significant ally. North and South Korea technically have remained in a state of war since three years of combat in the early 1950's ground to a stalemate.