President Lee Myung-bak was responding to the shelling of an inhabited island close to a disputed maritime border.
The South returned fire in one of the worst clashes since the Korean War.
US President Barack Obama called the incident an “outrageous, provocative act” by Pyongyang.
He was speaking ahead of an expected telephone call to President Lee.
The South Korean military had been carrying out an exercise near Yeonpyeong island, and the North accused the South of opening the hostilities – something Seoul denies.