South Korea has found the bodies of two civilians on the island of Yeonpyeong that North Korea attacked Tuesday, bringing the death toll to four.
The victims were construction workers who were burned in the fires that followed the North's artillery barrage.
"It's clearly a line for people, and crossing that line puts it in a different category," said John Delury, an assistant professor at Seoul's Yonsei University Graduate School of International Studies. "People here are feeling very conflicted, outrage and sorrow."
Two marines were also killed, and people visited South Korea's military hospital to pay their respects to the fallen soldiers. The U.S. and South Korea will hold joint military exercises in coming days.
Meanwhile, President Barack Obama emphasized the White House's vow to "stand shoulder to shoulder" with Seoul.
"South Korea is our ally," Obama told ABC News. "It has been since the Korean war, and we strongly affirm our commitment to defend South Korea as part of that alliance."
The USS George Washington aircraft carrier has left Japan to join the exercises in South Korea.
The United Nations called the skirmish between the two Koreas one of the "gravest incidents" since the end of the Korean War.